use earthly for helm testing instead of gitub actions

[djeebus]

worse? better? you be the judge!

pros:

• dockerfile-like syntax, easy to pick up
• avoid github action and the commit/push/wait/find error/resolve error/repeat CI cycle
• easily executable locally

cons:

• one more tool (only necessary if you want to test locally though)
• one more file format to learn (but mostly docker? mostly?)
• requires some extra start up (23 seconds vs 32 seconds for this build)

[mplachter]

Reduces the GitHub actions I'm a fan. 👍