fix(test): pre-generate node identity in external-signer test

The 04-external-signer integration test was flaky because the node
would spend too long generating an identity file, exceeding the 90s
RPC readiness timeout. Pre-inject the identity like other tests do.

Fixes #955

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vch9

test/integration/cli-tester/tests/signatory/04-external-signer.sh

@@ -32,6 +32,8 @@ om install-node \
 	--service-user tezos \
 	--no-enable 2>&1
 
+inject_identity "$NODE_INSTANCE"
+
 echo "==> Step 2: Start node to enable baker installation"
 om instance "$NODE_INSTANCE" start 2>&1