CI: Concurrency group

[pmikolajczyk41]

Currently, CI workflow will continue to run even if a new commit has been pushed to a PR branch / master. For simple Rust checks and tests this usually is just wasting resources. By introducing:

concurrency:
  group: ${{ github.ref }}-${{ github.workflow }}
  cancel-in-progress: true

(see: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency) we ensure that for every branch (github.ref) the Rust workflow (github.workflow) will be run only for the newest change - the previous launches will be canceled.

[jsdw]

Ooh sounds awesome; anything to speed up CI is a win for me!