Re-enable Windows CI unit tests with memory optimizations

[Claude]

Windows CI was disabled in #6930 due to MSVC running out of heap space (C1060: compiler is out of heap space) when building unit tests. This was caused by excessive parallel compilation processes consuming available memory on the CI runner.

Changes

• CMakeLists.txt: Add /MP2 flag for MSVC to limit parallel compilation to 2 processes per invocation
• Windows CI workflow:
  • Add --parallel 1 to cmake build commands to enforce sequential project builds
  • Add -A x64 flag to explicitly use 64-bit toolchain
  • Re-enable test building and execution

Approach

Two-level memory control:

1. Compiler level (/MP2): Limits MSVC to 2 parallel translation units per compilation
2. Build level (--parallel 1): Forces sequential target builds across the project

This reduces peak memory usage while maintaining some parallelism within individual compilation steps.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Windows CI job goes out of memory when building tests</issue_title>
<issue_description>We had to disable running unit tests on Windows in this PR. MSVC goes out of memory when trying to build tests:

2024-06-08T09:06:17.5884670Z D:\a\osrm-backend\osrm-backend\unit_tests\library\route.cpp(128,49): error C1060: compiler is out of heap space [D:\a\osrm-backend\osrm-backend\build\unit_tests\library-tests.vcxproj]

It seems it started happening after yet another update of CI base image.</issue_description>

<agent_instructions>Create a PR that re enables the unit test build on windows. Reduce parallel compilation — lower /MP degree (e.g. /MP2 instead of /MP) so fewer cl.exe processes compete for RAM simultaneously. Or define non parallel build in yaml:

• name: Build
  run: cmake --build build --parallel 1

Verify we are using the 64 bit toolchain.

</agent_instructions>

Comments on the Issue (you are @claude[agent] in this section)

• Fixes Windows CI job goes out of memory when building tests #6935