Use MSVC toolchain if PLATFORM is defined
<!-- Please describe your changes on the following line: -->
`VSInstallDir` exists only when Visual Studio is installed.
`VS140COMNTOOLS` is defined by installing either Visual Studio or [Visual C++ Build Tools](http://landinghub.visualstudio.com/visual-cpp-build-tools)
It will allow to build MSVC based Servo without having to install whole Visual Studio (few GiB).
EDIT: Another (maybe cleaner) solution would be reverting 63ec96a57d
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because `VS140COMNTOOLS` variable is already used in [mach.bat](https://github.com/servo/servo/blob/master/mach.bat)
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14346)
<!-- Reviewable:end -->
Instead of once per day (or night) with a date in the URL,
Cargo binaries are now built and published for every merge to master
(with the merge commit hash in the URL).
Fix test-wpt and test-css for Windows.
<!-- Please describe your changes on the following line: -->
In addition to minor changes for Windows, this forces Windows Python to
be used for all Windows builds (instead of using Windows Python only for
pc-windows-msvc builds).
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because testing the tests is too meta for me
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14039)
<!-- Reviewable:end -->
In addition to minor changes for Windows, this forces Windows Python to
be used for all Windows builds (instead of using Windows Python only for
pc-windows-msvc builds).
