internal/rosa: use stage3 mirror

These get taken down periodically and causes way too many rebuilds when they are taken down. Use mirror until a more elegant solution is available.

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

internal/rosa/rosa.go

@@ -210,14 +210,12 @@ func (t Toolchain) New(
 
 	case toolchainStage3:
 		name += "-boot"
-		var version, checksum string
+		var seed string
 		switch runtime.GOARCH {
 		case "amd64":
-			version = "20260111T160052Z"
-			checksum = "c5_FwMnRN8RZpTdBLGYkL4RR8ampdaZN2JbkgrFLe8-QHQAVQy08APVvIL6eT7KW"
+			seed = "c5_FwMnRN8RZpTdBLGYkL4RR8ampdaZN2JbkgrFLe8-QHQAVQy08APVvIL6eT7KW"
 		case "arm64":
-			version = "20260125T234618Z"
-			checksum = "79uRbRI44PyknQQ9RlFUQrwqplup7vImiIk6klefL8TN-fT42TXMS_v4XszwexCb"
+			seed = "79uRbRI44PyknQQ9RlFUQrwqplup7vImiIk6klefL8TN-fT42TXMS_v4XszwexCb"
 
 		default:
 			panic("unsupported target " + runtime.GOARCH)
@@ -226,16 +224,14 @@ func (t Toolchain) New(
 		args[0] = "bash"
 		support = slices.Concat([]pkg.Artifact{
 			cureEtc{},
-			toolchainBusybox.New("stage3-"+version, 0, nil, nil, nil, `
+			toolchainBusybox.New("stage3", 0, nil, nil, nil, `
 tar -C /work -xf /usr/src/stage3.tar.xz
 rm -rf /work/dev/ /work/proc/
 ln -vs ../usr/bin /work/bin
 `, pkg.Path(AbsUsrSrc.Append("stage3.tar.xz"), false,
 				pkg.NewHTTPGet(
-					nil, "https://distfiles.gentoo.org/releases/"+
-						runtime.GOARCH+"/autobuilds/"+version+
-						"/stage3-"+runtime.GOARCH+"-musl-llvm-"+version+".tar.xz",
-					mustDecode(checksum),
+					nil, "https://basement.gensokyo.uk/seed/"+seed,
+					mustDecode(seed),
 				),
 			)),
 		}, extra)