action: handle slashes in ref names

[woodruffw]

Fixes #62.

cc @MatthiasValvekens

[tnytown]

LGTM!

[tnytown]

CI may be failing because of the recent change in the staging root.

[woodruffw]

Hmm, not sure why TUF is failing here:

Run /home/runner/work/gh-action-sigstore-python/gh-action-sigstore-python/.//action.py "./test/artifact.txt"
Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/__main__.py", line 22, in <module>
    main()
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/_cli.py", line 542, in main
    _sign(args)
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/_cli.py", line 636, in _sign
    signer = Signer.staging()
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/sign.py", line 113, in staging
    rekor = RekorClient.staging(updater)
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/_internal/rekor/client.py", line 296, in staging
    rekor_keys = updater.get_rekor_keys()
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/_internal/tuf.py", line 178, in get_rekor_keys
    keys = self._get("Rekor", ["Active"])
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/_internal/tuf.py", line 143, in _get
    self._updater = self._setup()
  File "/home/runner/.local/lib/python3.10/site-packages/sigstore/_internal/tuf.py", line 137, in _setup
    updater.refresh()
  File "/home/runner/.local/lib/python3.10/site-packages/tuf/ngclient/updater.py", line [132](https://github.com/sigstore/gh-action-sigstore-python/actions/runs/4774812030/jobs/8488732941#step:3:135), in refresh
    self._load_root()
  File "/home/runner/.local/lib/python3.10/site-packages/tuf/ngclient/updater.py", line 323, in _load_root
    self._trusted_set.update_root(data)
  File "/home/runner/.local/lib/python3.10/site-packages/tuf/ngclient/_internal/trusted_metadata_set.py", line 164, in update_root
    self.root.verify_delegate(Root.type, new_root)
  File "/home/runner/.local/lib/python3.10/site-packages/tuf/api/metadata.py", line 452, in verify_delegate
    raise exceptions.UnsignedMetadataError(
tuf.api.exceptions.UnsignedMetadataError: root was signed by 0/1 keys

I tried an equivalent sigstore sign locally, which succeeds. Will keep debugging.

CC @jku for potential opinions 🙂

[woodruffw]

This looks like it might be more fallout from the underlying cause of sigstore/sigstore-python#594; CC @tnytown

[woodruffw]

The release in sigstore/sigstore-python#621 should fix this.

[tetsuo-cpp]

LGTM!