Workaround copr runtime dependency repo

Followup fedora-llvm-team#670. Reimplement without using a specific dnf.
Replace the unexpanded $distname variable in the script

tests/snapshot-gating.fmf

@@ -10,13 +10,17 @@
 
 summary: LLVM Tests for snapshot gating
 prepare:
-  # dnf5's copr plugin has trouble resolving the runtime dependency repos, so
-  # we need to use dnf4. Check https://github.com/fedora-copr/copr/issues/3387
   - name: Enable copr repo
     how: shell
-    script: |
-      dnf install -y dnf-plugins-core
-      dnf-3 -y copr enable @fedora-llvm-team/$COPR_PROJECT $COPR_CHROOT
+    script:
+      - dnf install -y 'dnf5-command(copr)' || dnf install -y 'dnf-command(copr)'
+      - dnf -y copr enable @fedora-llvm-team/$COPR_PROJECT $COPR_CHROOT
+      # dnf5's copr plugin has trouble resolving the runtime dependency repos,
+      # and is not replacing the $distname special var. Hence we need to do that
+      # parsing ourselves.
+      # Check https://github.com/fedora-copr/copr/issues/3387
+      - sed -i "s/\$distname/$(echo "$COPR_CHROOT" | cut -d '-' -f 1)/" /etc/yum.repos.d/*$COPR_PROJECT*
+
   # Lower the priority of the testing-farm-tag-repository so that our copr repo is picked up.
   # See: https://docs.testing-farm.io/Testing%20Farm/0.1/test-environment.html#_tag_repository
   - how: shell