fix windows build issue #18055
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With some reasons, the windows and VS environment do not set the `env("PLATFORM")` variable.
So, there's no need to check the variable, and set `os_type` equl to `pc-windows-msvc` as default. Then the script can get correct url to download `rustc`.
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix#18055 (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____
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