Revert "fix(sg): resolve overwrite env ordering in sg (#63838)" (#63924)

https://github.com/sourcegraph/sourcegraph/pull/63838 accidentally broke
the local-dev site configuration workflow (the site-configuration from
local dev was replaced with a stock configuration).

 Reverting this is probably the fastest way to resolve this. 

See https://sourcegraph.slack.com/archives/C07KZF47K/p1721329375844059
for more context.

## Test plan

CI

## Changelog

- The flawed env overwriting logic in sg from
https://github.com/sourcegraph/sourcegraph/pull/63838 has been reverted.

Author: ggilmore

dev/sg/internal/run/BUILD.bazel

@@ -43,7 +43,6 @@ go_test(
     srcs = [
         "docker_command_test.go",
         "logger_test.go",
-        "run_test.go",
     ],
     embed = [":run"],
     tags = [TAG_INFRA_DEVINFRA],

dev/sg/internal/run/command.go

@@ -265,7 +265,7 @@ type outputOptions struct {
 }
 
 func startSgCmd(ctx context.Context, cmd SGConfigCommand, parentEnv map[string]string) (*startedCmd, error) {
-	ex, err := cmd.GetExecCmd(ctx)
+	exec, err := cmd.GetExecCmd(ctx)
 	if err != nil {
 		return nil, err
 	}
@@ -279,12 +279,9 @@ func startSgCmd(ctx context.Context, cmd SGConfigCommand, parentEnv map[string]s
 	}
 
 	opts := commandOptions{
-		name: conf.Name,
-		exec: ex,
-		// The ordering here is quite important because we want to allow the `parentEnv` and `secretsEnv`
-		// to override the config values for a command. This ensures env vars in the sg overwrite file
-		// are always the final authority.
-		env:    makeEnv(conf.Env, parentEnv, secretsEnv),
+		name:   conf.Name,
+		exec:   exec,
+		env:    makeEnv(parentEnv, secretsEnv, conf.Env),
 		dir:    conf.RepositoryRoot,
 		stdout: outputOptions{ignore: conf.IgnoreStdout},
 		stderr: outputOptions{ignore: conf.IgnoreStderr},