coreclr pri-1 test build failures

[BruceForstall]

Due to PR #41947, all coreclr pri-1 test builds are failing

https://dev.azure.com/dnceng/public/_build/results?buildId=809051&view=results

There are many thousands of warnings (that seems new), but only 6 errors, including:

##[error]src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.cs(341,6): error CS8896: 'UnmanagedCallersOnly' can only be applied to ordinary static methods or static local functions.
/__w/1/s/src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.cs(439,6): error CS8895: Methods attributed with 'UnmanagedCallersOnly' cannot have generic type parameters and cannot be declared in a generic type. [/__w/1/s/src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.csproj]
##[error]src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.cs(439,6): error CS8895: Methods attributed with 'UnmanagedCallersOnly' cannot have generic type parameters and cannot be declared in a generic type.
/__w/1/s/src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.cs(440,48): error CS8894: Cannot use 'T' as a parameter type on a method attributed with 'UnmanagedCallersOnly'. [/__w/1/s/src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.csproj]
##[error]src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.cs(440,48): error CS8894: Cannot use 'T' as a parameter type on a method attributed with 'UnmanagedCallersOnly'.
/__w/1/s/src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.cs(520,10): error CS8895: Methods attributed with 'UnmanagedCallersOnly' cannot have generic type parameters and cannot be declared in a generic type. [/__w/1/s/src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.csproj]
##[error]src/tests/Interop/UnmanagedCallersOnly/UnmanagedCallersOnlyTest.cs(520,10): error CS8895: Methods attributed with 'UnmanagedCallersOnly' cannot have generic type parameters and cannot be declared in a generic type.

[jkotas]

Fixed by #41947