Fix environment variable name in find module

Fix required for XPRESS 8.13 using Bourne shell

FindXpress.cmake

 #adapted from SCIPOPT
 find_path(XPRESS_INCLUDE_DIRS
     xprs.h
-    HINTS ${XPRESS_DIR} $ENV{XPRESS_DIR}
+    HINTS ${XPRESS_DIR} $ENV{XPRESSDIR}
     PATH_SUFFIXES include)
 
 # todo: enable recursive search
 find_library(XPRESS_LIBRARY
     NAMES xprs
-    HINTS ${XPRESS_DIR} $ENV{XPRESS_DIR}
+    HINTS ${XPRESS_DIR} $ENV{XPRESSDIR}
     PATH_SUFFIXES lib)
 
 find_library(XPRESS_LIBRARY_cpp
     NAMES xprb
-    HINTS ${XPRESS_DIR} $ENV{XPRESS_DIR}
+    HINTS ${XPRESS_DIR} $ENV{XPRESSDIR}
     PATH_SUFFIXES lib)