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/git.go

@@ -17,14 +17,14 @@ func (t Toolchain) NewViaGit(
 	url, rev string,
 	checksum pkg.Checksum,
 ) pkg.Artifact {
-	return t.NewPackage(strings.TrimSuffix(
+	return t.New(strings.TrimSuffix(
 		path.Base(url),
 		".git",
 	)+"-src", path.Base(rev), nil, &PackageAttr{
 		KnownChecksum: &checksum,
 
-		Flag:  THostNet,
-		Paths: []pkg.ExecPath{resolvconf()},
+		HostNet: true,
+		Paths:   []pkg.ExecPath{resolvconf()},
 	}, &GenericHelper{
 		Build: `
 git \