.NET Framework build appears broken

[vcsjones]

I am seeing this build failure in PRs, such as #130875. From the build log

D:\a\_work\1\s\src\libraries\Microsoft.Extensions.Caching.Abstractions\tests\TrimmingTests\HybridCacheEntryOptionsAccessorTest.cs(17,6): error CS0246: The type or namespace name 'UnsafeAccessorAttribute' could not be found (are you missing a using directive or an assembly reference?) [D:\a\_work\1\s\src\libraries\Microsoft.Extensions.Caching.Abstractions\tests\Microsoft.Extensions.Caching.Abstractions.Tests.csproj::TargetFramework=net481]
D:\a\_work\1\s\src\libraries\Microsoft.Extensions.Caching.Abstractions\tests\TrimmingTests\HybridCacheEntryOptionsAccessorTest.cs(17,6): error CS0246: The type or namespace name 'UnsafeAccessor' could not be found (are you missing a using directive or an assembly reference?) [D:\a\_work\1\s\src\libraries\Microsoft.Extensions.Caching.Abstractions\tests\Microsoft.Extensions.Caching.Abstractions.Tests.csproj::TargetFramework=net481]
D:\a\_work\1\s\src\libraries\Microsoft.Extensions.Caching.Abstractions\tests\TrimmingTests\HybridCacheEntryOptionsAccessorTest.cs(17,21): error CS0103: The name 'UnsafeAccessorKind' does not exist in the current context [D:\a\_work\1\s\src\libraries\Microsoft.Extensions.Caching.Abstractions\tests\Microsoft.Extensions.Caching.Abstractions.Tests.csproj::TargetFramework=net481]
    0 Warning(s)
    3 Error(s)

This appears to have been introduced in #130403 cc @svick

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