ci: disable caching of the wheels that get built to save space

I had a pile of these one-shot wheels cached on the mac, taking about 1.2G of
space from the few weeks we've been building wheels to test, and with the few
times the macOS tests were actually run.  There's not much point in caching
these, since the tests run from a wheel file the tests are force fed.  There is
still an issue with one wheel from the tests being cached (probably on install),
but that's 1/6 of the original problem.

I'm putting this in the `pyproject.toml` because I think that's where we should
have all of the `cibuildwheel` config collected.  There's a concurrent effort to
modernize `setup.py` and use this file more, so I'll hold off on pushing most of
the config in here for now.  But this applies the setting to all OSes, as well
as when run through either CI or the individual buid scripts, so I think this is
clearly the right way to go.

Note that the docs for `cibuildwheel` talks about the intention to move from
`pip` to `build` some day.  This config only covers the former, so the other
config example[1] with the latter may be needed at some point.  But I don't see
an obvious option for that tool, so we'll cross that bridge when we come to it.

[1] https://cibuildwheel.pypa.io/en/v2.22.0/options/#examples_6

pyproject.toml

@@ -52,3 +52,9 build/
 '''
 skip-string-normalization = true
 quiet = true
+
+
+[tool.cibuildwheel]
+# Don't stockpile wheels in the pip cache directory when they get built, since
+# there's no mechanism to age old ones out.
+build-frontend = { name = "pip", args = ["--no-cache-dir"] }