Fix testing-farm-tag-repository priority lowering

Before trying to modify the repo we check that it exists, hence we no
longer need to mask the result of commands. This now works with both
dnf4 and dnf5

Fixes #671

Author: Jesus Checa Hidalgo

tests/snapshot-gating.fmf

@@ -25,8 +25,11 @@ prepare:
   # See: https://docs.testing-farm.io/Testing%20Farm/0.1/test-environment.html#_tag_repository
   - how: shell
     script: |
-      dnf install -y dnf-plugins-core
-      dnf config-manager --save --setopt="testing-farm-tag-repository.priority=999" || true
+      if dnf repolist | grep -q testing-farm-tag-repository; then
+        dnf install -y 'dnf5-command(config-manager)' || dnf install -y 'dnf-command(config-manager)'
+        dnf config-manager --save --setopt="testing-farm-tag-repository.priority=999" || \
+          dnf config-manager setopt "testing-farm-tag-repository.priority=999"
+      fi
 
   - name: Assert correct llvm package is installed
     how: shell