brew CI: install with --HEAD if necessary

[scpeters]

Needed by osrf/homebrew-simulation#3287.

Testing stable formulae

These should still work and not apply the --HEAD flag

• https://build.osrfoundation.org/job/gz_utils4-install_bottle-homebrew-arm64/173/
• https://build.osrfoundation.org/job/gz_utils-ci-gz-utils4-homebrew-arm64/47/

Testing with rotary prototype formulae

These should apply the --HEAD flag

• https://build.osrfoundation.org/job/gz_utils4-install_bottle-homebrew-arm64/175/
• https://build.osrfoundation.org/job/gz_cmake-ci-pr_any-homebrew-arm64/32/ (from CI test of "Bump main to 6.0.0~pre1" gazebosim/gz-cmake#530)
• it's not easy to test Bump main to 5.0.0~pre1 gazebosim/gz-utils#203 until Bump main to 6.0.0~pre1 gazebosim/gz-cmake#511 is merged, but I think this is enough testing for now