gitlab-ci: add cache, extend timeouts/retries to stabilize Fedora job

This is happening too often:
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: Error downloading packages:
  Curl error (28): Timeout was reached for https://mirrors.fedoraproject.org/metalink?repo=updates-released-f29&arch=x86_64 [Connection timed out after 30001 milliseconds]
ERROR: Job failed: exit code 1

https://gitlab.com/fdroid/fdroidserver/-/jobs/152719443

# Conflicts:
#	.gitlab-ci.yml

.gitlab-ci.yml

@@ -155,7 +155,12 @@ fedora_latest:
   only:
     - master@fdroid/fdroidserver
   script:
-    - dnf -y update
+    # tricks to hopefully make runs more reliable
+    - echo "timeout=600" >> /etc/dnf/dnf.conf
+    - echo "retries=50" >> /etc/dnf/dnf.conf
+    - echo "keepcache=True" >> /etc/dnf/dnf.conf
+
+    - dnf -y update || dnf -y update
     - dnf -y install findutils
                      git
                      gnupg