builder: a clone may offer the forge's credential, through git's own store

A private repository could not be built: the builder clones anonymously,
and had no way to say who it is. It already holds exactly one credential
to exactly the right place — the package-registry binding and its sealed
secret, one gitea user whose password answers npm and git alike — so a
clone now offers that, and nothing new is minted or carried.

Offered, never pushed: the credential is written as a git
credential-store file (0600, in the workspace, never argv) and named
with -c credential.helper, so git itself decides when it applies — only
on an authentication challenge, and only for the URL it was written
for, scheme, host and port included. A public repository clones exactly
as before; a repository on any other host is never shown it. The same
store rides along on an artifact's own context clone, so a private
module with a private context builds too.

cmd/mesh-builder/once.go

@@ -89,6 +89,7 @@ func buildOnce(ctx context.Context, args []string) error {
 		return err
 	}
 	built, buildErr := builder.Build(ctx, builder.Command, publisher, repository, *path, *ref, where, bases, npmrc,
+		forgeFrom(),
 		func(step, message string) { fmt.Fprintf(os.Stderr, "  [%s] %s\n", step, message) })
 	if buildErr != nil {
 		return buildErr