Explicitly install python with --overwrite (#2393)

```
==> Pouring python@3.11--3.11.0.monterey.bottle.tar.gz
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.

.github/workflows/tests.yaml

@@ -68,6 +68,11 @@ jobs:
         run: |
           echo '*** Updating brew'
           brew update
+          echo '*** Installing Python'
+          # Explicitly use --overwrite because of file conflicts in
+          # /usr/local/bin with GitHub's installed version. If this stops being
+          # required, great!
+          brew install --force-bottle --overwrite python@3.11
           echo '*** Installing LLVM deps'
           brew install --force-bottle --only-dependencies llvm
           echo '*** Installing LLVM itself'