ci: test eigen5 only on x64_64-linux + aarch64-darwin

scipy is not building yet with eigen5 on aarch64-linux

Author: nim65s

.github/workflows/nix.yml

@@ -21,14 +21,29 @@ jobs:
         with:
           name: gepetto
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-      - run: nix build -L .#proxsuite .#proxsuite-eigen_5
+      - run: nix build -L .#proxsuite
+
+  nix-eigen5:
+    runs-on: "${{ matrix.os }}"
+    strategy:
+      matrix:
+        os: ["ubuntu-24.04", "macos-15"]
+    steps:
+      - uses: actions/checkout@v5
+      - uses: cachix/install-nix-action@v31
+      - uses: cachix/cachix-action@v16
+        with:
+          name: gepetto
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+      - run: nix build -L .#proxsuite-eigen_5
 
   check:
     if: always()
     name: check-macos-linux-nix
 
     needs:
       - nix
+      - nix-eigen5
 
     runs-on: Ubuntu-latest