Internal improvement: Migrate Travis-based tests to GitHub Actions

[haltman-at]

This is me attempting to migrate our remaining Travis-based tests to GitHub actions.

We'll, uh, see whether it works?

[haltman-at]

OK, I'm going to try removing the truffle obtain step, on the basis that it's probably not necessary?

[haltman-at]

Hm, so:

1. The coverage build is failing due to that indexOf problem. I'll put in the hacky workaround for that, I guess, seeing as we don't currently know how to fix that.
2. Those two failing builds are supposed to be optional, but GitHub is counting the build as failing anyway. I'll, uh, have to figure out how to fix that...? I thought I it was just continue-on-error, but...

[haltman-at]

Ugh, the fricking decoder tests, with their stupid npxs! Of course those are ignoring the version of Truffle present here... we really need to do something about those (#3310).

Well, I guess I can make the debugger thing a separate PR...?

[haltman-at]

I'm gonna convert this to draft until after next release, I think.

[haltman-at]

Ugh, turns out GHA actually doesn't yet have a notion of allowed failures: actions/runner#2347 :-/

I guess I'll just close this PR for now.