internal/rosa: replace busybox dash with mksh

Toybox does not provide a shell, mksh fills that gap.

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

internal/rosa/ninja.go

@@ -10,6 +10,7 @@ func (t Toolchain) newNinja() pkg.Artifact {
 	return t.New("ninja-"+version, false, []pkg.Artifact{
 		t.Load(CMake),
 		t.Load(Python),
+		t.Load(Bash),
 	}, nil, nil, `
 chmod -R +w /usr/src/ninja/
 mkdir -p /work/system/bin/ && cd /work/system/bin/