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/strace.go

@@ -11,8 +11,6 @@ func (t Toolchain) newStrace() (pkg.Artifact, string) {
 		nil, "https://strace.io/files/"+version+"/strace-"+version+".tar.xz",
 		mustDecode(checksum),
 	), &PackageAttr{
-		SourceKind: SourceKindTarXZ,
-
 		// patch not possible with nonzero SourceKind
 		ScriptEarly: `
 sed -i 's/off64_t/off_t/g' \