CustomChecks unit tests getting possible concurency issue with bootstrap

[JanKrivanek]

Context

stage1\bin\bootstrap\core\sdk\9.0.200-preview.0.24603.3\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(221,5): error : (NETCORE_ENGINEERING_TELEMETRY=Build) The process cannot access the file 'D:\a\1\s\artifacts\bin\Microsoft.Build.BuildCheck.UnitTests\CustomChecks\Microsoft.Build.Framework.17.13.0-ci-24605-01.nupkg' because it is being used by another process.

https://dev.azure.com/dnceng-public/public/_build/results?buildId=887302&view=logs&j=20125c81-7edb-551b-693a-61efae016b74&t=7ce25265-aabe-59d3-d73c-06619cfcc3c4&l=256

[YuliiaKovalova]

I don't get it why it even happens, since we generate packages when build is done.
Or I misunderstand it

msbuild/eng/BootStrapMsBuild.targets

Line 242 in 82c8f93

<Import Project=".\TestAssets.targets" />

[ViktorHofer]

This affects the VMR stability significantly as it fails every ~70-100th build. The VMR pipeline builds ~70 legs per run which means that this is causing every second pipeline run to fail. I would appreciate if someone could take a look at this as soon as possible. Thanks.