Improve speed in workflow build-related operations about memo

[matuella]

@ggirotto what do you think about this?

[ggirotto]

I have a strong hypothesys about speeding up the release workflow. Today we build the iOS app twice: first by using flutter build and second by using build_ios_app fastlane action. I'm almost sure that we could just remove the build_ios_app fastlane action and keep just flutter build. I've notes on my side to test it, but I haven't done yet.

In the pull_request workflow, we could definitely cache flutter SDK. I've tried for a few hours, but without success I gave up because this github actions task were taking to long, but now I think we could go back and improve this caching stuff
Cache reference: subosito/flutter-action#16

[matuella]

I think #98 resulted in a decent performance improvement (I guess it was around ~33% of less time spent passing the workflow).

@ggirotto Are there other improvements that comes to your mind in both pull_request and release workflows that we can keep track, or do you think the time vs benefits are not worth to further improve the total time spent?

[ggirotto]

I had another hypothesis about caching the Flutter setup, but since the time taken to setup Flutter env is of only ~50 seconds I don't think it will bring great benefits to the overall build time.

[matuella]

Closing as we've reached a decent improvement with #98, although we can still look forward to more improvements if they appear plausible