internal/rosa: self-host stage0 tarball

This replaces gentoo stage3 tarballs.

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

internal/rosa/toybox.go

@@ -16,7 +16,9 @@ func (t Toolchain) newToybox(suffix, script string) pkg.Artifact {
 	), nil, stage0Concat(t, []string{},
 		"ROSA_CHECK=make USER=cure tests",
 	), `
-ln -s ../system/bin/bash /bin/ || true
+chmod +w /bin/
+ln -sf ../system/bin/bash /bin/
+
 cd /usr/src/toybox
 chmod +w kconfig tests
 rm \