Simplify CI workflow to standard haskell-ci workflow

[sgraf812]

The simpler, the better.

[sgraf812]

I'm not completely sure what changed since I tried something similar in #279, but now the vanilla CI workflow generated by haskell-ci just works.

It seems that our previous logic in tests/Makefile to set HAPPY was simply too smart for its own good.

I would be happy for @andreasabel to take a brief look, because he came up with the setup in #196. I verified that we indeed test the correct happy version which adding a dummy Makefile target path.run which outputs

--> Printing happy path...
which happy
/__w/happy/happy/dist-newstyle/build/x86_64-linux/ghc-8.6.5/happy-2.0/x/happy/build/happy/happy

[andreasabel]

Excellent!
Seem that time has made my workaround superfluous.
(Also, this was a few years back, now I have a better understanding of all the cabal workings and pitfalls.)

[sgraf812]

Thanks. I think your approach was necessary back when we still had the +bootstrap flag, but that was removed in #277.