Only load ReFrame programming env in find_modules if it is not yet in module cache

[casparvl]

Small optimization: we don't need to load the ReFrame programming environment if the entry is already in the module cache

[laraPPr]

lgtm

[casparvl]

For posterity: I tested using time reframe -c test-suite/eessi/testsuite/tests/apps/walberla/walberla.py -c test-suite/eessi/testsuite/tests/apps/MetalWalls.py --list and this PR reduced the runtime from 45 to 35 seconds. That's as expected, as on this cluster I saw the loading of the environment take 10s for every test class. In this case, it has to load them for walberla (first test class) since there's no module cache yet. But you save on it the second time around, thus saving 10 seconds.