Support .NET 5.0 pipe security.

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[ccruden-shookiot]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[ccruden-shookiot]

Seems to be building and testing mostly properly at this point.... Failing CancelServerStreaming in the GitHub build, but the comment in code seems to indicate that test is flaky, and it doesn't fail when I run it locally...

[cyanfish]

Also ignore the test error around .NET core not being available, that's a preexisting issue that must've been caused by a change to the github CI environment.