Create script that extracts the correct version from a commit tag

[spypsy]

There is a check we perform in multiple places in build-system, which is used to take a COMMIT_TAG, check if it's specific to a repo (e.g. aztec-node/v0.1.0-alpha33) and whether we're building tha repo, checks if it's a valid semver, and finally extract the version (e.g. 0.1.0-alpha33).
We should create an external function (extract_version) that can be used in all these places