Auto merge of rust-lang#128091 - heiher:xz-only, r=Mark-Simulacrum

build-manifest: Allow building manifests for formats that only have xz compression

Author: bors

src/tools/build-manifest/src/manifest.rs

@@ -88,7 +88,7 @@ impl Target {
         let gz = tarball_variant(builder, &base_path, "gz");
         let xz = tarball_variant(builder, &base_path, "xz");
 
-        if gz.is_none() {
+        if gz.is_none() && xz.is_none() {
             return Self::unavailable();
         }