Fix static dynamic test

This fixes two of the major failing cases for link/sungeun/static_dynamic.

Cygwin: This test depends on the file command displaying the link status of the
executable.  However, cygwin's files are in PE format - the file command does
not display many details about this format and I wasn't seeing an option that
would enable it to display correctly.  So, skip it here.
PrgEnv-*: The cray programming environments link statically by default, as
opposed to our usual environments which default to dynamic.  The prediff for
this test attempted to take that into account, but was checking against the
wrong environment variable to do so.  Fixing that.