Allow pre-existing stake accounts in multinode-demo/delegate-stake.sh (#10635)

(cherry picked from commit ae0d5ba201)

Co-authored-by: Trent Nelson <trent@solana.com>

multinode-demo/delegate-stake.sh

@@ -88,16 +88,15 @@ if [[ ! -f $vote_account ]]; then
   exit 1
 fi
 
-if [[ -f $stake_account ]]; then
-  echo "Error: $stake_account already exists"
-  exit 1
-fi
-
 if ((airdrops_enabled)); then
   $solana_cli "${common_args[@]}" airdrop "$stake_sol"
 fi
 
+if ! [[ -f "$stake_account" ]]; then
   $solana_keygen new --no-passphrase -so "$stake_account"
+else
+  echo "$stake_account already exists! Using it"
+fi
 
 set -x
 $solana_cli "${common_args[@]}" \