upload_ghc_libs: More control over which packages to operate on

Here we add a `--skip` flag to `upload_ghc_libs`, making it easier to
limit which packages to upload. This is often necessary when one package
is not uploadable (e.g. see #22740).

.gitlab/rel_eng/upload_ghc_libs.py

@@ -197,7 +197,7 @@ def main() -> None:
     parser_prepare.add_argument('--bindist', required=True, type=Path, help='extracted binary distribution')
 
     parser_upload = subparsers.add_parser('upload')
-    parser_upload.add_argument('--skip', nargs='*', type=str, help='skip uploading of the given package')
+    parser_upload.add_argument('--skip', default=[], action='append', type=str, help='skip uploading of the given package')
     parser_upload.add_argument('--docs', required = True, type=Path, help='folder created by --prepare')
     parser_upload.add_argument('--publish', action='store_true', help='Publish Hackage packages instead of just uploading candidates')
     args = parser.parse_args()
@@ -212,7 +212,7 @@ def main() -> None:
     if args.command == "upload":
         for pkg_name in args.skip:
             assert pkg_name in PACKAGES
-        pkgs = pkgs - args.skip
+        pkgs = pkgs - set(args.skip)
 
     if args.command == "prepare":
         manifest = {}