Update github CI to use ${GITHUB_SHA::7} for build tag instead of main

[aknochow]

The Github action currently builds the operator using the main tag in the ci which could potentially cause issues if AlwaysPull was not the image policy, which by default it is not. Rather than change the default image policy, we can set the image tag dynamically to a shortened PR SHA ${GITHUB_SHA::7} as we do in 3ci.