So the signing tool limitation cannot be worked around, and I believe the signing tool fails during the legacy MSI build because it makes multiple attempts at authentication. The next release of the signing tool will let us set %AZURE_TOKEN_CREDENTIALS%=AzureCliCredential and it should only try one approach, but right now they're using an old library without that support. So we're still using a secret token for legacy builds (which unfortunately means it's still just as vulnerable, but at least we aren't accessing it in most other builds now).

Looks like the final test build passed as well, so this is probably the final form of this PR.