try to fix build on macOS (#2503)

.github/workflows/build_cmake.yml

@@ -24,19 +24,19 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Configure
-      working-directory: ${{ github.workspace }}/dlib/test
-      run: cmake . -B ${{ env.build_dir }}
-
-    - name: Build just tests
       working-directory: ${{ github.workspace }}/dlib/test
       run: |
         if [ "$RUNNER_OS" == "macOS" ]; then
           # MacOS machines often come with low quality BLAS libraries installed, so don't use those.
-          cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4 -DDLIB_USE_BLAS=0 -DDLIB_USE_LAPACK=0
+          cmake . -B ${{ env.build_dir }} -DDLIB_USE_BLAS=0 -DDLIB_USE_LAPACK=0
         else
-          cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+          cmake . -B ${{ env.build_dir }}
         fi;
 
+    - name: Build just tests
+      working-directory: ${{ github.workspace }}/dlib/test
+      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+
     - name: Test
       working-directory: ${{ github.workspace }}/dlib/test/${{ env.build_dir }}
       run: |