Github Actions: cache modules and only run when necessary (#197)

This PR does two things:

- Only run the tests when relevant files change.
- Cache the Go Modules directory between runs, so they don't have to redownload everything everytime (go.sum is the cache key). Pretty much straight from the examples: https://github.com/actions/cache/blob/master/examples.md#go---modules

.github/workflows/gofmt.yml

@@ -4,6 +4,9 @@ on:
     branches:
       - master
   pull_request:
+    paths:
+      - '.github/workflows/gofmt.yml'
+      - '**.go'
 jobs:
 
   gofmt: