Replace IFD with build-time read for -env job generation

The previous IFD approach forced cross-platform builds during evaluation,
which could cause issues when Hydra evaluates on x86_64-linux but needs
to evaluate aarch64-darwin shells.

This new approach reads the mkShell output at build time using cat,
avoiding both:
- IFD which forces builds during evaluation
- recursive-nix which is not supported on remote builders

The shell is now a normal build dependency, and its output (containing
"declare -x VAR=value" lines) is read at build time to construct the
wrapper script.

Author: angerman

flake.nix

@@ -208,58 +208,38 @@
             } "touch  $out";
           } // (pkgs.lib.mapAttrs' (name: drv:
             pkgs.lib.nameValuePair "${name}-env" (
-            # Use IFD (Import From Derivation) to read the shell's environment output
-            # at evaluation time. This avoids the need for recursive-nix which is not
-            # supported on remote builders.
+            # Build-time read approach: read the mkShell output at build time using cat.
+            # This avoids both:
+            # - IFD (Import From Derivation) which forces cross-platform builds during eval
+            # - recursive-nix which is not supported on remote builders
             #
-            # The mkShell output is a file containing "declare -x VAR=value" lines.
-            # We read it via IFD, filter out the self-referencing $out variable,
-            # and embed it directly in the wrapper script.
-            let
-              # Read the shell's output (triggers build at eval time via IFD)
-              rawEnvContent = builtins.readFile drv;
-              # Get the shell's store path to filter out the self-reference
-              shellStorePath = builtins.unsafeDiscardStringContext (toString drv);
-              # Filter out the $out self-reference to break the circular dependency
-              envContent = builtins.replaceStrings
-                [ "declare -x out=\"${shellStorePath}\"" ]
-                [ "declare -x out=\"/dev/null\"" ]
-                rawEnvContent;
-            # this needs to be linux.  It would be great if we could have this
-            # eval platform agnostic, but flakes don't permit this.  A the
-            # platform where we build the docker images is linux (github
-            # ubuntu runners), this needs to be evaluable on linux.
-            in (import nixpkgs { system = "x86_64-linux"; }).writeTextFile {
-              name = "devx";
-              executable = true;
+            # The shell (drv) is a normal build dependency. Its output contains
+            # "declare -x VAR=value" lines which we read at build time.
+            pkgs.runCommand "devx" {} ''
+              # Read the shell's output at build time
+              shellContent=$(cat ${drv})
+
+              # Filter out the $out self-reference to avoid circular dependency issues
+              filteredContent=$(echo "$shellContent" | grep -v 'declare -x out=')
+
+              # Create the wrapper script
               # We use nix-shell to invoke bash, to work around some shells being just too ancient.
-              # This primarily happens on macOS. But as the sourced env may expect a bash version
-              # current with the current nix, using nix-shell to launch the bash is probably the
-              # most reliable option.
-              text = ''
-                #! /usr/bin/env nix-shell
-                #! nix-shell -i bash -p bash
+              # This primarily happens on macOS.
+              cat > $out <<'SCRIPT'
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash -p bash
 
-                set -euo pipefail
+set -euo pipefail
+SCRIPT
 
-                # Set up the environment (embedded via IFD from mkShell output)
-                ${envContent}
+              # Append the filtered environment
+              echo "$filteredContent" >> $out
 
-                source "$1"
-              '';
-              meta = {
-                description = "DevX shell";
-                longDescription = ''
-                  The DevX shell is supposed to be used with GitHub Actions, and
-                  can be used by setting the default shell to:
+              # Append the source command
+              echo 'source "$1"' >> $out
 
-                    shell: devx {0}
-                '';
-                homepage = "https://github.com/input-output-hk/devx";
-                license = pkgs.lib.licenses.asl20;
-                platforms = pkgs.lib.platforms.unix;
-              };
-            })) devShells)
+              chmod +x $out
+            '')) devShells)
           // (pkgs.lib.mapAttrs' (name: drv:
             pkgs.lib.nameValuePair "${name}-plans" drv.plans) devShells);
         packages.cabalProjectLocal-static        = (import ./quirks.nix { pkgs = static-pkgs; static = true; }).template;