Some JitBench fixes (dotnet/coreclr#19479)

Some JitBench fixes (dotnet/coreclr#19479)

Some JitBench fixes

- Fixed `dotnet run` from the `unofficial_dotnet` folder after `Directory.Build.targets` was added to `coreclr/tests/src`. This file is auto-imported and was adding dependencies and properties for targets that were not necessary. Added files to the root JitBench folder to override.
- Updated JitBench to be able to run against netcoreapp3.0. I haven't changed the default yet, could consider doing so once there are no pending perf issues being looked into.
  - Added a `RetargetProjects()` function to the setup steps to replace `TargetFramework` and `RuntimeFrameworkVersion` values in the relevant `.csproj` files to run against the expected runtime. This reduces some dependency on other repos containing the actual perf projects such that we don't have to update every repo to support environment variables to set those values.
- Disabled tiering for the `MinOpts` config

Commit migrated from https://github.com/dotnet/coreclr/commit/72e9b8d589f114776e3f42058d721058b77133b9