drop dangerous environment settings

Jenkins builds should never be affected by environment variables that
the user may have inadvertantly set.  Drop PKG_CONFIG_PATH,
LD_LIBRARY_PATH, and LIBGL_DRIVERS_PATH before building projects.
Add snbgt2 tester to pool.

Dylan has set up a gt2 sandy bridge for test.  Add it to the build
spec so we can get results for this platform.