brew CI: check RTOOLS_BRANCH for ci_matching_branch

[scpeters]

We currently allow different branches of metadata repositories to be used in some cases when branches start with ci_matching_branch/, as documented in the Gazebo CI docs page. Currently macOS CI jobs only check the ghprbSourceBranch variable, which is to the source branch of a GitHub pull request. It is inconvenient to open an entire pull request just to trigger a CI build with custom metadata, so I've updated the logic to also check the RTOOLS_BRANCH for the ci_matching_branch/ prefix, which can be set when starting stable branch CI jobs.

example build results here

In order to test the effect of gazebosim/gz-sensors#623 on gz-sim, I updated the gz-rotary-sensors formula to use the PR branch in osrf/homebrew-simulation@856ded1 and created a gz-sim branch from main with matching name, made a dummy change to the readme, opened a draft PR gazebosim/gz-sim#3664, and the homebrew CI job triggered for that PR used the specified branch of gz-sensors

With the approach in this branch, it is much simpler, as I was able to trigger a gz_sim-ci-main-homebrew-arm64 branch using the release-tools branch from this PR, without pushing a dummy change to gz-sim or opening a dummy PR:

• https://build.osrfoundation.org/job/gz_sim-ci-main-homebrew-arm64/113/

18:29:49 + [[ -n '' ]]
18:29:49 + [[ -n ci_matching_branch/brew_rtools_ci_matching_branch ]]
18:29:49 + python3 ./scripts/jenkins-scripts/tools/detect_ci_matching_branch.py ci_matching_branch/brew_rtools_ci_matching_branch
18:29:49 ci_matching_branch/brew_rtools_ci_matching_branch matches ci_matching_branch/
18:29:49 + export CI_MATCHING_BRANCH=ci_matching_branch/brew_rtools_ci_matching_branch
18:29:49 + CI_MATCHING_BRANCH=ci_matching_branch/brew_rtools_ci_matching_branch
18:29:49 + [[ -n ci_matching_branch/brew_rtools_ci_matching_branch ]]
18:29:49 + echo '# BEGIN SECTION: trying to checkout branch ci_matching_branch/brew_rtools_ci_matching_branch from osrf/simulation'
 trying to checkout branch ci_matching_branch/brew_rtools_ci_matching_branch from osrf/simulation
18:29:49 # BEGIN SECTION: trying to checkout branch ci_matching_branch/brew_rtools_ci_matching_branch from osrf/simulation
18:29:49 ++ brew --repo osrf/simulation
18:29:49 + pushd /opt/homebrew/Library/Taps/osrf/homebrew-simulation
18:29:49 /opt/homebrew/Library/Taps/osrf/homebrew-simulation ~/jenkins-agent/workspace/gz_sim-ci-main-homebrew-arm64
18:29:49 + git fetch origin ci_matching_branch/brew_rtools_ci_matching_branch
18:29:50 From https://github.com/osrf/homebrew-simulation
18:29:50  * branch              ci_matching_branch/brew_rtools_ci_matching_branch -> FETCH_HEAD
18:29:50 + git checkout ci_matching_branch/brew_rtools_ci_matching_branch
18:29:50 Switched to a new branch 'ci_matching_branch/brew_rtools_ci_matching_branch'
18:29:50 branch 'ci_matching_branch/brew_rtools_ci_matching_branch' set up to track 'origin/ci_matching_branch/brew_rtools_ci_matching_branch'.
18:29:50 + popd

TODO link to gazebosim/docs update PR

[j-rivero]

Looks good to my eyes, I've created the docs PR in gazebosim/docs#699