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/rosa.go

@@ -189,14 +189,16 @@ func (t Toolchain) New(
 	const lcMessages = "LC_MESSAGES=C.UTF-8"
 
 	var (
-		path    = AbsSystem.Append("bin", "busybox")
-		args    = []string{"hush", absCureScript.String()}
+		path    = AbsSystem.Append("bin", "sh")
+		args    = []string{"mksh", absCureScript.String()}
 		support []pkg.Artifact
 	)
 	switch t {
 	case toolchainBusybox:
 		name += "-early"
 		support = slices.Concat([]pkg.Artifact{newBusyboxBin()}, extra)
+		path = AbsSystem.Append("bin", "busybox")
+		args[0] = "hush"
 		env = fixupEnviron(env, nil, "/system/bin")
 
 	case toolchainStage3:
@@ -252,6 +254,7 @@ ln -vs ../usr/bin /work/bin
 			compilerRT,
 			runtimes,
 			clang,
+			boot.Load(Mksh),
 			boot.Load(Busybox),
 		})
 		env = fixupEnviron(env, []string{