bazel: Updates for garden (minus ABI break)

[mjcarroll]

Rework #513 to not break ABI.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.67%. Comparing base (ea90ada) to head (4d6ef43). Report is 21 commits behind head on gz-physics6.

Files	Patch %	Lines
src/InstallationDirectories.cc	0.00%	1 Missing :warning:

Additional details and impacted files

@@               Coverage Diff               @@
##           gz-physics6     #598      +/-   ##
===============================================
+ Coverage        78.52%   78.67%   +0.14%     
===============================================
  Files              140      140              
  Lines             7670     7652      -18     
===============================================
- Hits              6023     6020       -3     
+ Misses            1647     1632      -15     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[mjcarroll]

Tested here: https://github.com/gazebosim/gz-bazel/pull/70

[mjcarroll]

@osrf-jenkins retest this please

[azeey]

@azeey to take a look at macOS failures.