CI Failure on patch release PRs if main branch contains different instrumentations than the release branch

[xrmx]

It looks like the workflow we are using for the contrib workflow is hardcoded to the main branch:

https://github.com/open-telemetry/opentelemetry-python/blob/main/.github/workflows/contrib.yml#L19

This is a problem if the release branch contains different instrumentations because it'll try to run tox environments that are not available there.

e.g. When preparing the release of 1.39.1 CI tried to run the anthropic tox environments that were added after 1.39.0.