feat: support HERMIT_PREPEND_PATH to prepend entries to PATH

docs/docs/usage/envars.md

@@ -17,6 +17,7 @@ present in an active environment are:
 |-----------|------|
 | `HERMIT_ENV` | Path to the active Hermit environment. |
 | `HERMIT_BIN` | Path to the active Hermit environment `bin` directory. |
+| `HERMIT_PREPEND_PATH` | If set, prepends its value to the front of `PATH` after all other environment operations are applied. Useful for ensuring specific directories always take precedence over Hermit-managed packages. |
 
 An empty environment might look something like the following:
 

env.go

@@ -1429,6 +1429,9 @@ func (e *Env) allEnvarOpsForPackages(runtimeDeps []*manifest.Package, targetPkg
 	}
 	ops = append(ops, e.localEnvarOps()...)
 	ops = append(ops, e.ephemeralEnvars...)
+	if prependPath := os.Getenv("HERMIT_PREPEND_PATH"); prependPath != "" {
+		ops = append(ops, &envars.Prepend{Name: "PATH", Value: prependPath})
+	}
 	return ops
 }
 

integration/integration_test.go

@@ -673,6 +673,15 @@ EOF
 				assert test "${CLEANUP_DONE:-}" = "yes"
 			`,
 		},
+		{
+			name:         "HermitPrependPathIsRespected",
+			preparations: prep{fixture("testenv1")},
+			script: `
+				export HERMIT_PREPEND_PATH="/prepend/first:/prepend/second"
+				. bin/activate-hermit
+				assert echo "$PATH" | grep -q "^/prepend/first:/prepend/second:"
+			`,
+		},
 		{
 			name:         "InstallOnActivateEnsuresPackagesAreUnpacked",
 			preparations: prep{fixture("testenv-install-on-activate"), activate(".")},