Cache artifacts in Travis, AppVeyor, GitHubActions, and Cirrus

[christopher-dG]

Closes #166
I've tested Travis, AppVeyor, and GHA, but I couldn't get Cirrus to pick up on my repo for some reason. According to the docs it should work.
GitLab doesn't allow caching files outside of the repository directory, so caching can't be used there. DroneCI's caching looks a bit more complicated so maybe I'll look at it another day, it's not used frequently anyways.

[codecov]

Codecov Report

Merging #173 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #173   +/-   ##
=======================================
  Coverage   89.54%   89.54%           
=======================================
  Files          18       18           
  Lines         507      507           
=======================================
  Hits          454      454           
  Misses         53       53           

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[giordano]

Awesome, thank you so much!

Cirrus is being very unreliable lately, many packages moved to Travis for FreeBSD testing. As far as I know, Cloud Drone doesn't support caching, there are plugins for self-hosted instances.

[christopher-dG]

How does FreeBSD on Travis work? And good to know that the Drone stuff will only work on self-hosted, so not much use implementing it here.

[giordano]

How does FreeBSD on Travis work?

Pretty good, just add

  - freebsd

to the list of operating systems, that's it

[christopher-dG]

It seems that it's not mentioned in the Travis docs anywhere...

[giordano]

I think it's in beta or something like that, but Alex shared the news on Slack 🙂

[christopher-dG]

Ah, cool. I'll wait until it's GA before I add the option here.