Use cache to speed-up go execution in tests and building

Hopefully using GOCACHE that is cached by the runners will speed-up a little the execution of tests and compilation.

Checked on the pipeline for this MR:

• first run (without cache) 👉 https://gitlab.com/gitlab-org/fleeting/fleeting-plugin-aws/-/jobs/3908323219

  Execution time: 1min 23sec

  step_script execution time: 1min 08sec

• second run (with cache populated) 👉 https://gitlab.com/gitlab-org/fleeting/fleeting-plugin-aws/-/jobs/3908331558

  Execution time: 40sec

  step_script execution time: 5sec

So ~51% speed-up of the overall job time and ~92% speed-up of the go test execution itself 😉

I'm curious how it will look for the compilation step 🙂