The workflow definition is adapted from gocryptfs,
that uses github actions for a month now.

Works ok as seen here:
https://github.com/rfjakob/go-fuse/actions/runs/1167431646

(except that some tests are flakey, but that's a
problem for another day).

Change-Id: Ib7aaa3b0a2f9671040fa599735aa47323d5c25d4