Added a README section on using GitHub Actions to run experiments with a private repo

[ribafish]

Added a README section on using GitHub Actions to run experiments with a private repo. It leverages the github actions checkout and passes that as a local git repo to the experiment actions.

This is needed as the scripts themselves are using git clone command, which will fail with a private repo since access is not configured in the env by default. This should fix errors like below, as well as explains how to do it where it's expected (instead of searching for this by issues)

Run gradle/gradle-enterprise-build-validation-scripts/.github/actions/gradle/experiment-1@actions-stable
Run # Read the action inputs

Cloning private-gha-tests
Cloning into '/home/runner/work/private-gha-tests/private-gha-tests/gradle-enterprise-gradle-build-validation/.data/01-validate-incremental-building/20240920T092821-66ed4035/build_private-gha-tests'...
fatal: could not read Username for 'https://github.com/': No such device or address
ERROR: Unable to clone git repository https://github.com/ribafish/private-gha-tests
Error: Process completed with exit code 100.