Don't require a GitHub token

[jku]

The new extremely-dangerous-public-oidc-beacon current-token branch allows us to fetch the JWT without a GitHub token. This makes local testing easy, which makes developing the suite easier.

Changes:

• git clone the beacon repo to get the token
• Do this in a loop until token is valid (95% of the time the first try succeeds)
• Cache the token so we only do this once in a test run
• Remove all mention of github token in arguments, comments and docs

Some additional notes:

• retry times are tweaked from the values upstream (and special values are set for the interactive case)
• The caching could be smarter (could return cached value only if it's valid) but maybe this simple thing works?

Release Note

• --github-token is no longer required to run signing tests

[jku]

This makes the test run significantly faster as well.

That is just a result of the caching which we could have done with the previous approach as well so it's kind of unrelated.