tools/build/make.py: Avoid calling brew --prefix on macOS unnecessarily

If all the require variables (XCC/XCXX/XCPP/XLD) are already set in the
environment, we don't have to infer a default value for the cross toolchain
path. This avoids an additional brew --prefix call when building with
cheribuild (since it already sets all these variables).

(cherry picked from commit 2b181156c893843266c2825098360db2364dbd23)