Fixes for gazebo-install-one_liner-homebrew-amd64

[scpeters]

I noticed the following failed CI job and made a few fixes after some debugging:

• https://build.osrfoundation.org/view/ign-citadel/job/gazebo-install-one_liner-homebrew-amd64/2959/

That first failure has the following error:

++ brew cleanup --prune-prefix
./scripts/jenkins-scripts/lib/_homebrew_cleanup.bash: line 34: brew: command not found

The fix in 8e85201 is to use ${BREW_BINARY} to match the rest of the script.

After that I saw a brew doctor complaint about sbin not being in the PATH:

• https://build.osrfoundation.org/view/ign-citadel/job/gazebo-install-one_liner-homebrew-amd64/2960/

Warning: Homebrew's "sbin" was not found in your PATH but you have installed
formulae that put executables in /usr/local/sbin.
Consider setting your PATH for example like so:
  echo 'export PATH="/usr/local/sbin:$PATH"' >> ~/.zshrc

The fix is in 115c708, copied from #931.

I then made a change in 70c04c4 and 46f55b9 to use a local copy of the script in CI, and after it passed, I reverted to curling the script and followed the recommendation to get it from https://get.gazebosim.org

https://build.osrfoundation.org/view/ign-citadel/job/gazebo-install-one_liner-homebrew-amd64/2963/

The CI job should be fixed once this is merged.

[j-rivero]

minor suggestion about curl parameters, looks good.