ci: run lambda workflow on all PRs, not just PRs to main

Remove branch restriction from lambda.yml workflow to support stacked PRs
and feature branch workflows. The workflow will now run on any PR that
modifies lambda code or the workflow itself.

Co-authored-by: npalm <11609620+npalm@users.noreply.github.com>

Author: Copilot

.github/workflows/lambda.yml

@@ -2,8 +2,6 @@ name: Build lambdas
 
 on:
   pull_request:
-    branches:
-      - main
     paths:
       - 'lambdas/**'
       - '.github/workflows/lambda.yml'