Pass C_INCLUDE_PATH to test environment

At least two or three of NAVs dependencies may need to be built from
source when pulled in by pip: psycopg2, python-ldap and Pillow.  On
platforms like NixOS, the include files for the necessary C libraries
may not be in standardized FHS locations, so the C_INCLUDE_PATH is used
to convey their locations to the C compiler.

tox.ini

@@ -45,6 +45,7 @@ setenv =
     django32: DJANGO_VER=32
     django40: DJANGO_VER=40
 passenv =
+    C_INCLUDE_PATH
     GITHUB_ACTIONS
     GITHUB_RUN_ID
     USER