.gitlab-ci.yml: use Fedora 33 image

Signed-off-by: Daiki Ueno <ueno@gnu.org>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 4a9f22a..56819f6 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,6 +1,6 @@
 variables:
   BUILD_IMAGES_PROJECT: gnutls/build-images
-  FEDORA_BUILD: buildenv-f31
+  FEDORA_BUILD: buildenv-fedora33
   DEBIAN_X86_CROSS_BUILD: buildenv-debian-x86-cross
   GET_SOURCES_ATTEMPTS: "3"