Bug 1724606 - Replace uses of MACOSX_DEPLOYMENT_TARGET env variable with -mmacosx-version-min flag. r=firefox-build-system-reviewers,andi

They are equivalent, except for the fact that MACOSX_DEPLOYMENT_TARGET
may apply to more processes in a normal build. In practice, all the
processes that matter are covered through compiler flags.

On the opposite end, MACOSX_DEPLOYMENT_TARGET isn't necessary passed in
all cases (like clangd, mach static-analysis, etc.), while compiler
flags are.

Differential Revision: https://phabricator.services.mozilla.com/D122145