internal/rosa/go: migrate to generic helper

The go toolchain predates all abstractions currently available. This migration causes rebuilds due to internal cleanups affecting the final build script.

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

internal/rosa/rosa.go

@@ -413,6 +413,8 @@ type PackageAttr struct {
 	// Passed to [Toolchain.NewPatchedSource].
 	Patches []KV
 
+	// Unregistered extras.
+	Extra []pkg.Artifact
 	// Passed through to [Toolchain.New], before source.
 	Paths []pkg.ExecPath
 	// Passed through to [Toolchain.New].
@@ -481,7 +483,8 @@ func (t Toolchain) NewPackage(
 		rn = name + "-" + version
 	}
 	wantsChmod, wantsWrite := helper.wantsChmod(), helper.wantsWrite()
-	extraRes := make([]pkg.Artifact, 0, 1<<3+len(extra))
+	extraRes := make([]pkg.Artifact, 0, 1<<3+len(attr.Extra)+len(extra))
+	extraRes = append(extraRes, attr.Extra...)
 	{
 		pv := paGet()
 		for _, p := range helper.extra(attr.Flag) {