Condition out internal authentication for VerifyDotnetFolderContents test

[lbussell]

Fixes #6007

Internal test run: https://dev.azure.com/dnceng/internal/_build/results?buildId=2568476&view=logs&j=1cd3c31d-f87a-5340-7f4c-1efc1ae5d874&t=e94617f8-c028-5efe-9cc4-f7765cc028e5

This was a regression from #5928

[NikolaMilosavljevic]

It's odd that #5928 did not hit this error in any of the checks. But, the fix seems correct.

[lbussell]

It's odd that #5928 did not hit this error in any of the checks. But, the fix seems correct.

#5928 may not have built any/many Dockerfiles due to caching. However, I can't say why #5989 didn't hit any issues.

[mthalman]

It's odd that #5928 did not hit this error in any of the checks. But, the fix seems correct.

#5928 may not have built any/many Dockerfiles due to caching. However, I can't say why #5989 didn't hit any issues.

Caching doesn't affect testing. We still run tests even if images are cached.