GitHub env file support

[torbjornvatn]

This PR attempts to add support for the new Environment Files since the ::set-env command has been deprecated.

I found a way to download a tar of the envs.txt file that I've assigned to $GITHUB_ENV from a previous step's container and merge the contents into the env map that is passed to the next step.
It's possible that there are more elegant ways of doing this, but I couldn't find any other way of getting access to that file between steps.

UPDATE: I did an new attempt at solving this using the existing command handler, but it didn't work as the new way of setting the envs uses the >> to file syntax. That output can be generated from any command that's called within the run: and as far as I understand that can't be intercepted in the command handler.

DEPRECATION? We should probably add a warning or an error if you try to use the old and deprecated ::set-env syntax, but I'm not sure how to best handle that @cplee

Fixes #384

[cplee]

@torbjornvatn - i really appreciate you working on this!

I'm going to need more time to review...lots of changes and it is a bit hard to discern which changes are related to #384 and which were just general "clean up" changes.

[dkravetz]

@bixu and I are also running into this issue. I'd be happy to test a release candidate 😃

[cplee]

@torbjornvatn - can you fix the conflict?

[torbjornvatn]

@cplee sorry for the delay!
Fixed the conflict with master now

[torbjornvatn]

@cplee I've also fixed the linting issue now.

[2m]

Thank you for this very much! I noticed that writing to GITHUB_PATH to add to system path does not currently work. However it is possible to work around it with:

echo "PATH=$PATH:$(python3 -m site --user-base)/bin" >> ${GITHUB_ENV}