Summary:
Rather than have a single main.yml file containing all off the different
builds, split that up so that we have one file per build environment
(linux, mac, windows).

This has a couple of advantages:

* It is quicker to see the status of just one of the platforms
* Artifact collection for one platform is not blocked pending completion
  of the builds for all systems
* It's a little easier to understand what is happening for a single platform

To support having multiple files I've changed the output-file option to
be an output-dir.

I've included the rm of main.yml in this commit.  Once this gets
imported back to the FB internal system I'll amend in an update to
the helper script that updates all of our opensource builds and run
and amend that.
Pull Request resolved: https://github.com/facebook/folly/pull/1360

Test Plan:
the GH action status on this PR should show three different
actions running, one for each platform.

I updated and ran
`fbcode/opensource/fbcode_builder/getdeps/facebook/update-all-github-actions.sh`
to regenerate all the actions files for FB.

Reviewed By: yfeldblum

Differential Revision: D21310991

Pulled By: wez

fbshipit-source-id: 604ef652c8f746781a4b410c6b996cdee4524e0d