.homebrew-build-env is not (eventually) idempotent

[dimpase]

Steps To Reproduce

On macOS with Homebrew, repeatedly running
.homebrew-build-env leads to unlimited growth of the environment variables it's setting, such as LIBRARY_PATH, PATH, PKG_CONFIG_PATH, etc. In extreme cases this appears to lead to weird build bugs, as reported on sage-devel

Expected Behavior

repeated running of . .homebrew-build-env should not lead to such a growth, it should stop on the 1st or 2nd repetition.

Actual Behavior

the growth does not stop.

Additional Information

No response

Environment

• OS: macOS + Homebrew
• Sage Version: 10.4+

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide