internal/rosa: remove legacy exec helper

This change also adds a fourth stage to the bootstrap machinery, offering more correct toolchain validation that works around LLVM dynamic linking flaws.

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

internal/rosa/go.go

@@ -13,7 +13,7 @@ func (t Toolchain) newGo(
 	script string,
 	boot ...pkg.Artifact,
 ) pkg.Artifact {
-	return t.NewPackage("go", version, newTar(
+	return t.New("go", version, newTar(
 		"https://go.dev/dl/go"+version+".src.tar.gz",
 		checksum,
 		pkg.Gzip,
@@ -75,7 +75,7 @@ func init() {
 		)
 		switch t.arch {
 		case "amd64":
-			bootstrapEarly = []pkg.Artifact{t.NewPackage("go", "1.4-bootstrap", newTar(
+			bootstrapEarly = []pkg.Artifact{t.New("go", "1.4-bootstrap", newTar(
 				"https://dl.google.com/go/go1.4-bootstrap-20171003.tar.gz",
 				"8o9JL_ToiQKadCTb04nvBDkp8O1xiWOolAxVEqaTGodieNe4lOFEjlOxN3bwwe23",
 				pkg.Gzip,