Run cross-compilation builds on every push

[nshalman]

What does this pull request do?

This creates a Github action to run cross-compilation
builds for all supported targets (or at least that's the idea)

This would have caught #389 and should
catch build failures for supported targets.

Where should the reviewer start?

The output of the builds

How should this be manually tested?

N/A

[nshalman]

Builds ran in my fork: https://github.com/nshalman/fsnotify/actions/runs/1709688309

[nshalman]

Even on my illumos system I could have caught that (which is why we should run these cross-compilation builds as part of CI):

~/fsnotify $ git checkout v1.5.0 ; GOOS=freebsd GOARCH=amd64 go build
Note: switching to 'v1.5.0'.
<snip>
HEAD is now at 08848a0 v1.5.0 preparation (#380)
# github.com/fsnotify/fsnotify
./kqueue.go:193:43: undefined: unix.O_SYMLINK

[nshalman]

I think this PR is a good example of how I intend to maintain quality in this project across all supported platforms. I hope you agree, @nathany.

[nathany]

very good idea