Update action to cache the gitops-pusher binary

[evilhamsterman]

Compiling the binary every time causes the action to take longer to run the job than it should. This updates the action to instead of using go run it uses go install and then caches the binary after it's built. On future runs the binary is restored from cache making the job run in only a few seconds versus minutes.

This should fix #56 at least the build time concern.