Fix SOPS azkv envCred

[apeschel]

At the moment, the envCred logic can't actually set the Azure credentials.

This commit fixes the logic so that the environment variables can actually be used to set the Azure credentials.

There are other issues that come up from this block of code, but those can be dealt with separately.

[apeschel]

I ran this locally and verified it fixes the issue. It does run into rate limiting with the Azure API, but that is a separate issue to solve.

[darkowlzz]

I did some investigation around the history of this code and found that it was added in #813 recently. The getDefaultAzureCredential() function is based on upstream implementation where instead of creating a ChainedTokenCredential, after iterating through all the possible auth methods, once auth material is found for any auth method, the creds are immediately returned.

The related issue #841 where you mention about the integration tests not working, it looks like that may be because of the MasterKey being from the upstream sops project and not the locally defined version of MasterKey, refer

kustomize-controller/internal/sops/azkv/keysource_integration_test.go

Line 130 in 09f4158

encryptKey := &azkv.MasterKey{

. The upstream sops MasterKey reads the env var creds properly and the test passes.
That can be fixed as part of this change or separately.

Update: Upon further investigation of the tests code, it looks like the test mentioned above it is a compatibility test and intentionally uses upstream SOPS to check if the local implementation is compatible with the upstream one. Which means we can add new test that reads env vars in order to test the broken auth.

For now, as it is, it LGTM!
I'd like to have another review+approval before merging this.