When running `python3 ./mach bootstrap-android`, this error occurs:
```
AttributeError: 'HTTPMessage' object has no attribute 'getheader'
File "/servo/python/servo/bootstrap_commands.py", line 136, in bootstrap_android
download("sdk", tools.format(system=system))
File "/servo/python/servo/bootstrap_commands.py", line 117, in download
download_file(filename, url, archive)
File "/servo/python/servo/util.py", line 170, in download_file
download(desc, src, fd)
File "/servo/python/servo/util.py", line 110, in download
if resp.info().getheader('Content-Length'):
```
Use the `get()` function instead, which exists in both Python 3 and 2
(where it is a synonym for `getheader()`).
Fixes#25616.
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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- [ ] These changes fix#18055 (github issue number if applicable).
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