internal/rosa: optionally load built-ins from filesystem

This avoids having to rebuild cmd/mbf over and over.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/builtins_env.go

@@ -0,0 +1,28 @@
+//go:build penv
+
+package rosa
+
+import (
+	"io/fs"
+	"os"
+)
+
+// builtinAzalea is the backing directory of built-in azalea-based [Artifact]
+// implementations.
+var builtinAzalea = func() fs.FS {
+	const name = "ROSA_SOURCE"
+
+	pathname, ok := os.LookupEnv(name)
+	if !ok {
+		println(name + " not set")
+		os.Exit(1)
+	}
+
+	root, err := os.OpenRoot(pathname)
+	if err != nil {
+		println(err.Error())
+		os.Exit(1)
+	}
+
+	return root.FS()
+}()