Bug 9541: make OPAC login page respect OPAC_CSS_OVERRIDE

When clicking the login link for opac-user.pl in a multiple branch
scenario the environment variable for OPAC_CSS_OVERRIDE was ignored from
the koha-conf.xml file.  It seems like is is working on every page in
the opac except for the login page.

Test Plan:
1) Set up a Koha server with 2 separate catalog configurations
   ( e.g. opac1.kohatest, opac2.kohatest )
2) Set the OPAC_CSS_OVERRIDE directive for separate css files
   in each opac
3) Browse to the opac login page, note the css is not applied
4) Apply this patch
5) Reload the page, note the css is now applied

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>