ci: skip build.yml on default branch pushes

`build.yml` triggers on `push` and `workflow_dispatch`. The reusable
`dappnode-build-hash.yml` SDK already auto-detects pushes to the default
branch and runs a no-op test build (no IPFS pin, no comment), so the
workflow run is wasted CI on every merge.

Add `branches-ignore: ["master"]` to the push trigger so the workflow
doesn't even start on default-branch pushes. `workflow_dispatch` and
pushes to other branches (e.g. tropibot bump branches) still trigger it
as before.

Author: 3alpha

.github/workflows/build.yml

@@ -3,6 +3,8 @@ name: Build
 on:
   workflow_dispatch:
   push:
+    branches-ignore:
+      - "master"
     paths-ignore: ["README.md"]
 
 jobs: