ci: run the Linux unit tests in Release

The unit-test matrix was the only test job still built in Debug, and the
Engine suite was taking 26-27 of its 30 minutes. In Debug the JIT does not
optimize PortableMath (64 ns per Sin against 17 ns for the runtime's), so
after 49603edd the suite reached the 30-minute limit and was cancelled
(run 42728099456). Every other test job (windows-desktop, the Windows
release build, the server image) already runs Release, which is what
ships; no production project has Debug-only code. The Windows release
workflow still runs OpenNest.Tests in Debug.

.github/workflows/ci.yml

@@ -32,7 +32,7 @@ jobs:
         with:
           dotnet-version: '8.0.x'
       - name: Run ${{ matrix.project }}
-        run: dotnet test "${{ matrix.project }}/${{ matrix.project }}.csproj"
+        run: dotnet test "${{ matrix.project }}/${{ matrix.project }}.csproj" -c Release
 
   synthetic-nests:
     name: Six synthetic Irregular nests