[Libomptarget] Fix block and thread limit environment variables not being respected

The next-gen plugins did not properly set the values from
`OMP_NUM_TEAMS` and `OMP_TEAMS_THREAD_LIMIT`. This is because these
maximum values are set by each plugin to its hardware maximum. This
happens *after* the previous initialization. Move it to the correct
place and then add a test.

Fixes https://github.com/llvm/llvm-project/issues/61082

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D145105