Summary:
This commit adds a getdeps command that is able to generate
a workflow file for the GitHub Actions CI environment.

The workflow file could be expressed more simply using the matrix
syntax and with three steps (checkout, build, test), but I chose to
break out the steps for each of the dependencies because the UX
while waiting on the build is much nicer that way: the steps show
during and live log tailing for the section of the build that is
underway.  If they were all lumped into a single build step then
the logs from the boost section of the build dominate and make
the github UI work very hard.

Pull Request resolved: https://github.com/facebook/watchman/pull/743

Test Plan:
https://github.com/facebook/watchman/pull/743 successfully
executes the github actions CI flow.

```
$ opensource/fbcode_builder/getdeps.py generate-github-actions --output-file watchman/.github/workflows/main.yml watchman
```

Reviewed By: simpkins

Differential Revision: D17384915

Pulled By: wez

fbshipit-source-id: 9a9e5a3e806c18f6cc38ba1cb7059740cda01ad4