internal/rosa: assume file source is xz-compressed

XZ happens to be the only widely-used format that is awful to deal with, everything else is natively supported.

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

internal/rosa/gnu.go

@@ -639,7 +639,6 @@ func (t Toolchain) newFindutils() (pkg.Artifact, string) {
 		nil, "https://ftpmirror.gnu.org/gnu/findutils/findutils-"+version+".tar.xz",
 		mustDecode(checksum),
 	), &PackageAttr{
-		SourceKind: SourceKindTarXZ,
 		ScriptEarly: `
 echo '#!/bin/sh' > gnulib-tests/test-c32ispunct.sh
 echo 'int main(){return 0;}' > tests/xargs/test-sigusr.c