Update CheckProjects to not fail for non-dev CORE_ROOT

Update CheckProjects to not fail for non-dev CORE_ROOT

CheckProjects expects to be able to find the jit test project files
relative to the value of the `CORE_ROOT` environment variable. This
fails when we have test setups like cross-tests or helix where the
test binary tree has been copied away from the repo after building.

Tolerate these cases by returning a success code if the `CORE_ROOT`
path is not pointing at the expected place.

Fixes dotnet/coreclr#11079.
Fixes dotnet/coreclr#10635.

Commit migrated from https://github.com/dotnet/coreclr/commit/6521f0772322420d9b9e8528e9c121131808749e