Replace recursive-nix with IFD for -env job generation

The -env jobs previously used recursive-nix to run `nix print-dev-env`
inside a derivation, but this fails on remote builders with:
  "recursive-nix is not supported yet by external derivation builders"

This commit replaces that approach with Import From Derivation (IFD):
- Read the mkShell output directly at evaluation time via builtins.readFile
- Filter out the self-referencing $out variable to break circular deps
- Embed the environment content directly in the wrapper script

The mkShell output already contains "declare -x VAR=value" lines which
is exactly what's needed. This approach:
- Avoids the recursive-nix requirement entirely
- Still includes all runtime dependencies (GHC, cabal, tools, etc.)
- Works on remote builders without special configuration

Author: angerman

flake.nix

@@ -208,16 +208,23 @@
             } "touch  $out";
           } // (pkgs.lib.mapAttrs' (name: drv:
             pkgs.lib.nameValuePair "${name}-env" (
-            # We need to use unsafeDiscardOutputDependency here, as it will otherwise
-            # pull in a bunch of dependenceis we don't care about at all from the .drvPath
-            # query.
-            let env = pkgs.runCommand "${name}-env.sh" {
-                requiredSystemFeatures = [ "recursive-nix" ];
-                nativeBuildInputs = [ pkgs.nix ];
-              } ''
-              nix --offline --extra-experimental-features "nix-command flakes" \
-                print-dev-env '${builtins.unsafeDiscardOutputDependency drv.drvPath}^*' >> $out
-            '';
+            # 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.
+            #
+            # 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
@@ -235,8 +242,9 @@
 
                 set -euo pipefail
 
-                # Set up the environment
-                source ${env}
+                # Set up the environment (embedded via IFD from mkShell output)
+                ${envContent}
+
                 source "$1"
               '';
               meta = {