CI: Set the path to php-fpm so it's found by proxy_fcgi.t (after r1918149).

Github: closes #453


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1918150 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 54dcd7b..0e42a0d 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -21,6 +21,8 @@
 env:
   MARGS: "-j2"
   CFLAGS: "-g"
+  # This will need updating as the ubuntu-latest image changes:
+  PHP_FPM: "/usr/sbin/php-fpm8.1"
 
 jobs:
   build: