unstableGitUpdater: Fix support for fetchFromGit{Hub,Lab}

When `fetchFromGitHub` uses `fetchzip` instead of `fetchgit` internally,
which is the most common use case, there is no `src.url` attribute
so the update will fail.

Let’s fix that by falling back to `src.meta.homepage`,
which these fetchers set to the repository URL.

pkgs/common-updater/unstable-updater.nix

@@ -21,7 +21,7 @@ let
     # By default we set url to src.url
     if [[ -z "$url" ]]; then
         url="$(${nix}/bin/nix-instantiate $systemArg --eval -E \
-                   "with import ./. {}; $UPDATE_NIX_ATTR_PATH.src.url" \
+                   "with import ./. {}; $UPDATE_NIX_ATTR_PATH.src.url or $UPDATE_NIX_ATTR_PATH.src.meta.homepage" \
             | tr -d '"')"
     fi