GHA: improve detection if branch exists on nrf upmerge

to check if branch exists we have to check

Signed-off-by: Robert Gałat <robert.galat@nordicsemi.no>

Author: RobertGalatNordic

.github/workflows/nrf-upmerge-pr.yml

@@ -46,7 +46,7 @@ jobs:
           git fetch --all
 
           # Check if PR branch exists
-          if git show-ref --verify --quiet refs/heads/$BRANCH_NAME; then
+          if git show-ref --verify --quiet refs/remotes/origin/$BRANCH_NAME; then
             echo "Branch $BRANCH_NAME exists, checking PR status..."
 
             # Check if PR exists and is open