tests: use `--no-cache-dir` with `pip`

After 1a09563a615c, there's one more wheel that gets cached in the user's pip
cache in the macOS CI runner.  The wheel corresponds to the version being used
for the tests, but it doesn't get cached until the 3rd or 4th test shard is run,
so it's not an issue with installing to run the tests.  This seems to eliminate
that.

This doesn't seem to be an issue on Windows or Linux in my setup.  Windows not
being affected is likely because we set `$USERPROFILE` to redirect the home
directory to `$TESTTMP` when running tests, since 08fd76a553c9.  (When checking
with `"$PYTHON" -m pip cache dir`, it points to `$TESTTMP/pip/cache`.)  We do
also set `$HOME` to this same location when running posix tests, but I can't
tell what's going on locally in Linux, because running `pip` directly in the *.t
explodes, and `"$PYTHON" -m pip --version` prints `pip 9.0.1 from /usr/lib/python3/dist-packages`,
so that's likely before caching was enabled[1].  Running `python3.8 -m pip --version`
locally outside of the *.t (the same version used to invoke the test runner), prints
`pip 24.2 from /home/mharbison/.local/lib/python3.8/site-packages/pip (python 3.8)`.

In CI, both macOS and Linux print a modern version of `pip`, and list the cache
as being under `$TESTTMP`, but then it doesn't end up there on macOS.  No idea
if it is a pip bug, or what.  But let's be explict and disable caching.

[1] https://github.com/pypa/pip/blob/fe0925b3c00bf8956a0d33408df692ac364217d4/docs/html/topics/caching.md?plain=1#L37

tests/test-install.t

@@ -214,7 +214,7 our virtual env. Then pip doesn't think
 
 Note: we use this weird path to run pip and hg to avoid platform differences,
 since it's bin on most platforms but Scripts on Windows.
-  $ (syshgenv; ./installenv/*/pip install $TESTDIR/.. >> pip.log)
+  $ (syshgenv; ./installenv/*/pip --no-cache-dir install $TESTDIR/.. >> pip.log)
     Failed building wheel for mercurial (?)
   WARNING: You are using pip version *; however, version * is available. (glob) (?)
   You should consider upgrading via the '$TESTTMP/installenv/bin/python* -m pip install --upgrade pip' command. (glob) (?)