Explicitly install python with --overwrite

[jonmeow]

==> Pouring [email protected]
Error: The brew link step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/2to3-3.11
Target /usr/local/bin/2to3-3.11
already exists. You may want to remove it:
  rm '/usr/local/bin/2to3-3.11'

Installing llvm with --overwrite didn't seem to pass it on dependencies, particularly python (https://github.com/carbon-language/carbon-lang/actions/runs/3464174572/jobs/5785378508). So this instead installs python separately, and then --overwrite works.

See https://github.com/carbon-language/carbon-lang/actions/runs/3464364789/jobs/5785802783 for the example passing run. The actions on this PR will probably still use the trunk version.