internal/rosa: consistent stage0 paths

This makes using the gentoo stage3 archive as our stage0 compatible with

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

internal/rosa/rosa.go

@@ -173,7 +173,7 @@ func fixupEnviron(env, extras []string, paths ...string) []string {
 
 // absCureScript is the absolute pathname [Toolchain.New] places the fixed-up
 // build script under.
-var absCureScript = fhs.AbsUsrBin.Append(".cure-script")
+var absCureScript = AbsSystem.Append("bin", ".cure-script")
 
 const (
 	// TExclusive denotes an exclusive [pkg.Artifact].
@@ -187,7 +187,7 @@ func (t Toolchain) New(
 	name string,
 	flag int,
 	extra []pkg.Artifact,
-	checksum *pkg.Checksum,
+	knownChecksum *pkg.Checksum,
 	env []string,
 	script string,
 
@@ -220,14 +220,14 @@ func (t Toolchain) New(
 		default:
 			panic("unsupported target " + runtime.GOARCH)
 		}
-		path = fhs.AbsRoot.Append("bin", "bash")
-		args[0] = "bash"
 		support = slices.Concat([]pkg.Artifact{
 			cureEtc{},
 			toolchainBusybox.New("stage0", 0, nil, nil, nil, `
 tar -C /work -xf /usr/src/stage0.tar.xz
 rm -rf /work/dev/ /work/proc/
 ln -vs ../usr/bin /work/bin
+mkdir -vp /work/system/bin
+ln -vs ../../bin/sh /work/system/bin
 `, pkg.Path(AbsUsrSrc.Append("stage0.tar.xz"), false,
 				pkg.NewHTTPGet(
 					nil, "https://basement.gensokyo.uk/seed/"+seed,
@@ -279,7 +279,7 @@ ln -vs ../usr/bin /work/bin
 	}
 
 	return pkg.NewExec(
-		name, checksum, pkg.ExecTimeoutMax, flag&TExclusive != 0,
+		name, knownChecksum, pkg.ExecTimeoutMax, flag&TExclusive != 0,
 		fhs.AbsRoot, env,
 		path, args,