update-plugin-list: use json, use http caching, use gh action caches

[RonnyPfannschmidt]

this change

• switches from html index usage to json index usage
• adds using requests-cache as http cache to trim down runtime
• adds gh action caches for pip and the new http cache

[nicoddemus]

This is interesting, but I wonder if we need to speed up this workflow at all? This runs only once a week, and looking at the list of runs, each run takes only around 4m or less.

[RonnyPfannschmidt]

When I hacked on it to use json, the feedback time was a pain, adding the cache made it a breeze to run

[nicoddemus]

Got it, thanks!

[RonnyPfannschmidt]

fixing the false cache-hit issue wrt last-serial made it more complicated, lets reeval

[bluetech]

fixing the false cache-hit issue wrt last-serial made it more complicated, lets reeval

Didn't realize requests-cache ignores the cache-control header by default. The fix LGTM.

[RonnyPfannschmidt]

@bluetech it's not that it ignores it, it's just that the default cache controls lead to stale data, one needs to compare extra data to force a refresh

None of the http standard caching tools enable the required data pass over, so manual intervention is needed