tex does not support paths that contain tildes, and tmpdir may be such a
path. Fortunately, we can arrange to refer to paths relatively instead.

No test, as that would involve setting up another MPLCONFIGDIR just for
that purpose, which is rather costly as we'd need to regen the font
cache for it. But one can try e.g.

MPLCONFIGDIR=/tmp/has~tilde python -c 'from pylab import *; figtext(.5, .5, "a", usetex=True); show()'

Closes #21375 (comment).

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).