CI: add package build checks for all flake packages

Adds a 'packages' job to the Nix workflow that builds scip, speedtest,
and proto-generate to catch build failures on PRs.

Author: jupblb

.github/workflows/nix-checks.yaml

@@ -21,3 +21,17 @@ jobs:
       - uses: DeterminateSystems/nix-installer-action@v21
       - uses: DeterminateSystems/magic-nix-cache-action@v13
       - run: nix build .#checks.x86_64-linux.${{ matrix.check }}
+
+  packages:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        package:
+          - scip
+          - speedtest
+          - proto-generate
+    steps:
+      - uses: actions/checkout@v6
+      - uses: DeterminateSystems/nix-installer-action@v21
+      - uses: DeterminateSystems/magic-nix-cache-action@v13
+      - run: nix build .#${{ matrix.package }}

flake.nix

@@ -58,6 +58,8 @@
               wrapProgram $out/bin/speedtest \
                 --set SCIP_SAMPLE_INDEXES_DIR ${sampleIndexes}
             '';
+
+            meta.mainProgram = "speedtest";
           };
 
           proto-generate =