tests-via-crave.yml - switch to the correct branch for the PR

[uvatbc]

Description

PRs have remote refs.
That remote ref will not be automatically fetched when we fetch origin.

Solution

So how do we fetch it?

1. first fetch the actual remote reference and map it to a "local ref". This is done using git fetch origin <remote-ref>:<name for local ref>
2. Then we switch to the ref: git checkout <name for local ref>

This sets the state of the local clone to what the PR expects it to be

Tests

I've run this on a private repository with Crave's internal self-hosted Actions.

Needs to run against the solr GHA to test

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide