services: nginx: Pass run-directory on config test.

Fixes <https://bugs.gnu.org/36388>.
This fixes error logging errors in the activation script.

* gnu/services/web.scm (nginx-activation): Add "-p" flag.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/web.scm

@@ -682,6 +682,7 @@ of index files."
        (system* (string-append #$nginx "/sbin/nginx")
                 "-c" #$(or file
                            (default-nginx-config config))
+                "-p" #$run-directory
                 "-t"))))
 
 (define (nginx-shepherd-service config)