github: Add virtualenv caching

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.
• I've updated CHANGELOG.md if applicable.
• I've added tests applicable for this pull request

What does this Pull Request accomplish?

Cache virtualenvs used in GitHub Actions workflows.

Why should this Pull Request be merged?

Speed up PR/CI workflows, reduce GitHub/PyPI resource usage.

What testing has been done?

TBD

[github-actions]

Test Results

    34 files  ±0      34 suites  ±0   58m 9s ⏱️ + 1m 51s
 2 396 tests ±0   2 042 ✅ ±0    354 💤 ±0  0 ❌ ±0 
43 102 runs  ±0  36 850 ✅ ±0  6 252 💤 ±0  0 ❌ ±0 

Results for commit 4b5aabb. ± Comparison against base commit a5cc5bf.

♻️ This comment has been updated with latest results.

[bkeryan]

1st run: https://github.com/ni/nidaqmx-python/actions/runs/14451634791/job/40525509094?pr=702

Build - 1m 4s
Run unit tests (ubuntu-latest, 3.9) - 49s
Run unit tests (windows-latest, 3.9) - 1m 22s
Generate docs - 52s
Run system tests (win-10-py64) - 35m 13s

[bkeryan]

Caching on the self-hosted runner isn't working yet because it doesn't have zstandard installed:

1s
Post job cleanup.
"C:\Program Files\Git\usr\bin\tar.exe" --posix -cf cache.tgz --exclude cache.tgz -P -C C:/ar/_work/nidaqmx-python/nidaqmx-python --files-from manifest.txt --force-local -z
/bin/sh: line [1](https://github.com/ni/nidaqmx-python/actions/runs/14451634791/job/40525716518?pr=702#step:17:1): gzip: command not found
/usr/bin/tar: cache.tgz: Cannot write: Broken pipe
/usr/bin/tar: Child returned status 1[2](https://github.com/ni/nidaqmx-python/actions/runs/14451634791/job/40525716518?pr=702#step:17:2)7
/usr/bin/tar: Error is not recoverable: exiting now
Warning: Failed to save: "C:\Program failed with error: The process 'C:\Program Files\Git\usr\bin\tar.exe' failed with exit code 2

[bkeryan]

1st run: https://github.com/ni/nidaqmx-python/actions/runs/14451634791/job/40525509094?pr=702

Build - 1m 4s
Run unit tests (ubuntu-latest, 3.9) - 49s
Run unit tests (windows-latest, 3.9) - 1m 22s
Generate docs - 52s
Run system tests (win-10-py64) - 35m 13s

3rd run:

Build - 1m 4s (cached)
Run unit tests (ubuntu-latest, 3.9) - 55s (cached)
Run unit tests (windows-latest, 3.9) - 1m 17s (cached)
Generate docs - 40s (cached)
Run system tests (win-10-py64) - 38m 47s (not cached, uploaded cache)

4th run:

Build - 1m 10s (cached)
Run unit tests (ubuntu-latest, 3.9) - 51s (cached)
Run unit tests (windows-latest, 3.9) - 1m 21s (cached)
Generate docs - 52s (cached)
Run system tests (win-10-py64) - 37m 4s (cached)

For the "fast" workflows like "Build" and "Run unit tests", there is one more venv we're not caching yet: the one in which Poetry is installed. The setup-poetry action is still downloading the Poetry install script and Poetry packages every time.

For the "slow" workflows ("Run system tests"), the caching is working but it takes ~4 min to upload or download the cache file, so it's not really a win (with respect to overall time) at this point. However, at least it should be accessing PyPI less.