Work around pypy38: skipped error, in newer tox and uv, in CI

Skipping missing interpreters became an error in newer versions of
tox-uv in particular, in 1 test runner, Windows PyPy 3.8. Other test
runners were unaffected. I'm not sure if the 1 affected test runner's
interpreter is actually missing. I suspect it's a bad check on the newer
tool versions' part. Either way, interpreters should _not_ be missing
and skipped in CI. _Not_ skipping missing interpreters causes CI to
succeed.

.github/workflows/ci.yml

@@ -46,3 +46,4 @@ jobs:
         run: tox
         env:
           TOXENV: ${{ matrix.language.toxenv }}
+          TOX_OVERRIDE: tox.skip_missing_interpreters=false