gitlab-ci: Set LC_ALL=C.UTF-8

Although piglit could now handle non-ASCII characters in the
environment, meson was still failing without this (even though it's
using Python 3).

Reviewed-by: Eric Anholt <eric@anholt.net>
(Cherry picked from commit bc6998b728)

.gitlab-ci.yml

@@ -56,6 +56,7 @@ build-and-test:
         paths:
             - build/test/piglit-results/
     variables:
+        LC_ALL: C.UTF-8
         PIGLIT_DIR: /root/piglit
         XTEST_DIR: /root/xts
     script: