gha: delete azure RG only if it exists

delete_cluster() has tried to delete the az resources group regardless
if it exists. In some cases the result of that operation is ignored,
i.e., fail to resource group not found, but the log messages get a
little dirty. Let's delete the RG only if it exists then.

Fixes #8989
Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>

tests/gha-run-k8s-common.sh

@@ -104,10 +104,14 @@ function get_cluster_credentials() {
 
 function delete_cluster() {
     test_type="${1:-k8s}"
-
-    az group delete \
-        -g "$(_print_rg_name ${test_type})" \
-        --yes
+    local rg
+    rg="$(_print_rg_name ${test_type})"
+
+    if [ "$(az group exists -n "${rg}")" == "true" ]; then
+        az group delete \
+            -g "${rg}" \
+            --yes
+    fi
 }
 
 function delete_cluster_kcli() {