Call SetDllDirectory("") after sqlite3 init on cygwin

Cygwin sqlite3 is patched to call SetDllDirectory("/usr/bin") on init, which
affects the current process and is inherited by child processes.  It causes
DLLs to be loaded from /usr/bin/ before $PATH, which breaks all sorts of
things.  A typical failures would be header/lib version mismatches (e.g.
openssl when running checkPhase on openssh).  We'll just set it back to the
default value.

Note that this is a problem with the cygwin version of sqlite3 (currently
3.18.0).  nixpkgs doesn't have the problematic patch.