Fixes #4502

The unnoticed mistake was that BASH doesn't match anything in brace expansions if there's only one element.

$ bash -c 'ls dist/*{.tar.gz}'
ls: cannot access 'dist/*{.tar.gz}': No such file or directory
$ bash -c 'ls dist/*{.tar.gz,.exe}'
ls: cannot access 'dist/*.exe': No such file or directory
 dist/streamlink-4.0.0.tar.gz
$ bash -c 'shopt -s nullglob; ls dist/*{.tar.gz,.exe}'
dist/streamlink-4.0.0.tar.gz

As an alternative to this PR, I've got #4497 pretty much ready and validated multiple test releases in my test-repo using my test-account, but just apply this simple fix here and release 4.0.1.

Issue was introduced by #4405:
2dd2f35 #diff-7829468e86c1cc5d5133195b5cb48e1ff6c75e3e9203777f6b2e379d9e4882b3R153