ci: Run CI on all branches and pin CFD library version

- Remove branch restriction to run CI on any push (not just main/master)
- Add CFD_VERSION env variable to pin CFD C library to v0.1.5
- Use ref parameter to checkout specific CFD release tag

Author: shaia

.github/workflows/build-wheels.yml

@@ -2,11 +2,14 @@ name: Build and Test Wheels
 
 on:
   push:
-    branches: [main, master]
   pull_request:
   workflow_dispatch:
   workflow_call:
 
+env:
+  # CFD C library version to build against
+  CFD_VERSION: "v0.1.5"
+
 jobs:
   build_wheel:
     name: Build wheel on ${{ matrix.os }}
@@ -24,6 +27,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: ${{ github.repository_owner }}/cfd
+          ref: ${{ env.CFD_VERSION }}
           path: cfd
           fetch-depth: 0