Add IMPORT, use symlinks, and use default build commands

This PR adds three features to the entrypoint.sh script.

It allows env={IMPORT="root/repo"} for cases where a repository's import name isn't the same as its repo name on GitHub, as is often the case when you fork a golang project.
It uses ln -s instead of cp -R, to save time and space when building large repos.
It provides make (if there's a Makefile) or go build && go test as default commands. In many cases the user won't have to specify args.

So the simplest usage is now:

action "ci" {
  uses="cedrickring/golang-action@1.1.0"
}

I've made each of the three additions its own commit(s), in case you want to cherry-pick and choose.

cedrickring / golang-action