AzureContainerRegistry tests failing

[jeremymeng]

may be related to CAE change? Nightly started failing on the same day CAE changes went in.

https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4308167&view=results

Live tests also failing: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4358042&view=logs&j=43701d7e-216c-59bf-08d1-1efd0c7ac90a&t=837467d0-f383-5ba6-1b1a-21aa68346dfb&l=138

[jeremymeng]

/cc @minhanh-phan @timovv

[timovv]

Almost certainly related to CAE. We may need to do what Key Vault did and roll out a custom policy for ACR. It's concerning that this actually caused things to break though -- I was under the impression that we would need extra work in ACR to enable CAE support, but the existing policy should still function. Maybe there's a bug in Core where we aren't dealing with custom challenge handlers properly.