Use git describe for finding operator version

[rhmdnd]

We need to be able to set the replaces attribute of the CSV file so that OLM can build a relationship between dependencies, which is important for disconnected installations and upgrades.

Previously, we were using the version in the CSV file, which worked but it would change within the make prepare-release target depending on when the command was invoked.

This commit updates the version script to use git describe instead, which makes finding the current version more robust since tagging isn't done until a subsequent step of the release process.