[Bug]: Excessively expensive reuse action

[dkfellows]

What happened?

Processing of workflows using the reuse action is currently relatively expensive, mostly due to the cost of installing the tool (and its dependencies).
The reuse action should be altered to use a pre-built Docker image of the action so that we can bear the (especially time) cost of installation once instead of every time someone commits to a PR branch.

What version of our actions are you running?

main

Which runner were you using?

ubuntu-latest

Relevant log output

[rowleya]

That is how the default action works, but I'm not that sure about the process to get that working, so I will defer this to someone who has time to look at it!

[dkfellows]

I suspect that building a docker image would be the way forward here. Then we could make the action use that. (Note that this is how the standard one really works...)

[dkfellows]

Hmm, I now understand what's going on. When adopting the reuse action, one often has many failing workflows to start with. Until one of the workflows passes, the cache isn't populated. (Oops!)

The fix is probably to expand getsentry/action-setup-venv@v2.2.0 (a composite action) and to add a use of actions/cache/save@v4.3.0 "at the end" of it to commit the cache. (It's a shame that the cache key isn't an output of the action-setup-venv action; that would let us commit the cache when we want without feeling the need to hack the action apart.) Or I could fully separate cache restoration... hmm...

[dkfellows]

There were two things required:

1. Splitting the caching so that the cache would be saved even if the workflow failed (due to adding a commit).
2. Defining more carefully what was to be examined for the cache key; some parts (such as the .git directory) are different on every checkout, despite not being interestingly different.

The result is an approximately halving of the execution time of the overall workflow add-license workflow. Not bad!