GH Actions: Cache pip dependencies

[MetRonnie]

This is a small change with no associated Issue. Hopefully will shave some time off GH Actions checks [Update: not really 🤷‍♂️]

https://github.blog/changelog/2021-11-23-github-actions-setup-python-now-supports-dependency-caching/

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Does not need tests
• No change log entry required
• No documentation update required.

[MetRonnie]

Don't know if there is a reason to avoid caching for the manylinux test?

[oliver-sanders]

Partially addresses #3606

[oliver-sanders]

Do we have the ability to reset this cache manually?

[MetRonnie]

https://github.com/actions/setup-python#caching-packages-dependencies says:

Please Note: Restored cache will not be used if the requirements.txt file is not updated for a long time and a newer version of the dependency is available that can lead to an increase in total build time.

(It only talks about requirements.txt but actions/setup-python#282 (comment) says you can use setup files too)

---

To be honest, I'm minded to close this as it isn't really making a dent on install times

[oliver-sanders]

To be honest, I'm minded to close this as it isn't really making a dent on install times

Fine with me, the big cost ATM is the Conda envs.

I think we should containerise our build env and install deps from Conda so we are always testing the correct stack and aren't don't get messed around with CI env changes. We could build and cache the env based on the hash of the conda-environment.yml and setup.cfg files which would massively cut down on build time. (could also roll the Bash tests into the same container framework using Conda to install the different Bash versions rather than building our own).

See:

• swarm: use docker images for local tests #4007
• actions: remote testing functionality #3641
• actions: cache environment #3606