perf(CI): use unity builds to speed up Windows CI (XRPLF#4780)

The unity build speeds up compilation by bundling multiple source files
into one larger file. This reduces Windows CI build time by up to 50%.

As described in XRPLF#4596, the automatic Windows builds take a very long
time. Unity builds are significantly faster - currently about 45 min,
much closer to the typical MacOS (35-40 minutes) and nix (~30 minutes)
run times.

This is intended as a stopgap solution until a more resourced and
reliable runner is available.

No C++ code was changed. This only affects CI.

.github/workflows/windows.yml

@@ -89,7 +89,7 @@ jobs:
           generator: '${{ matrix.generator }}'
           configuration: ${{ matrix.configuration }}
           # Hard code for now. Move to the matrix if varied options are needed
-          cmake-args: '-Dassert=ON -Dreporting=OFF -Dunity=OFF'
+          cmake-args: '-Dassert=ON -Dreporting=OFF -Dunity=ON'
       - name: test (permitted to silently fail)
         shell: bash
         # Github runners are resource limited, which causes unit tests to fail