When writing pub credentials, ensure that the target paths exists before
trying to write the file. In the past this would have worked because the
credentials were in the pub cache, and the pub cache would exist by the
time the repo tooling code was running. The new location, however, is a
configuration directory that is unlikely to exist before anything
involving `pub` credentials has run.
Should fix the latest `release` failure:
https://github.com/flutter/packages/actions/runs/4949005383/jobs/8854219784
As of Dart 3.0 (Flutter 3.10), the location that the tool was using for
the pub cache wasn't being read, breaking autopublish in CI. This
updates to the new location, and adds tests for it.
Should fix the fact that the `release` step hangs on every package
change now.
