internal/rosa: install bash as sh

This works around software relying on bashisms even when explicitly invoking sh.

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

internal/rosa/gnu.go

@@ -187,6 +187,7 @@ cd "$(mktemp -d)"
 	--without-bash-malloc
 make "-j$(nproc)" check
 make DESTDIR=/work install
+ln -s bash /work/system/bin/sh
 `, pkg.Path(AbsUsrSrc.Append("bash"), true, pkg.NewHTTPGetTar(
 		nil,
 		"https://ftp.gnu.org/gnu/bash/bash-"+version+".tar.gz",