Add Github Actions CI using choosenim

[iffy]

This is yet another alternative for switching from Travis to Github Actions. It supersedes #871 and #868 with the following differences:

• It uses choosenim to test Nim versions 0.20.2, 1.0.10, 1.2.8 and 1.4.0 (so as choosenim improves, so does this)
• It also tests with Nim built from Git SHA 8a004e2
• It also tests with this nightly https://github.com/nim-lang/nightlies/releases/tag/2020-11-11-devel-bbe49a14ae827b6474d692042406716a3b3dd71f
• @genotrance requested "I would prefer one place where all this is solved, not yet another copy of code that has to be maintained in each repo." This does that by using the Github Action iffy/install-nim@v3 instead of including a script file.

You can see the build run for this PR here: https://github.com/iffy/nimble/actions/runs/370324359

I apologize for making 3 PRs for this issue, but they haven't followed a sequential pattern of development.

[dom96]

Cool. Do I dare look at the implementation of iffy/install-nim? :)

[dom96]

is this just a demo? :)

Pretty cool, but we don't need this here.

[iffy]

This mimics the current Travis build: https://github.com/nim-lang/nimble/blob/master/.travis.yml#L13 I'm fine removing it.

[dom96]

well, it's either this or the nightly. We don't need both. If the nightly works and is newer let's keep it instead of this.

[dom96]

I'm guessing choosenim is not used for this case?

[iffy]

Correct. If there's a something: prefix, choosenim is not used.

[iffy]

Cool. Do I dare look at the implementation of iffy/install-nim? :)

Nope, you'd better not :) The car runs -- don't pop the hood. (Hint: it's pretty much that script from the last PR)

[iffy]

Don't merge yet -- nightly isn't working. I'll fix it.

[iffy]

Okay, now nightly is building right: https://github.com/iffy/nimble/actions/runs/373098791

[dom96]

Epic, thanks!