Refactor CNCF Flux integration and e2e tests with Azure to use workload identity

Our CI is using static client secrets and Azure DevOps PATs, which we need to rotate yearly. They just expired and I'm now rotating them to mitigate the CI failures, but ideally we should get rid of this toil and configure Azure workload identity for GitHub Actions in our workflows (both flux2 and pkg). It's also a more secure way to protect our Azure account.