Auto merge of #140754 - dpaoliello:cygwinpath, r=marcoieni

[win][CI] Convert paths to Windows format before adding to PATH

While trying to get the `aarch64-msvc` build working correctly (#140136), I noticed that the `PATH` observed during the Rust steps of the build on Windows builds had some entries still in MinGW format, which means that Windows would not be able to use them correctly:

From <https://github.com/rust-lang/rust/actions/runs/14606167901/job/40975555680#step:28:192> note the path for `ninja` and `sccache`.

```
PATH = Some(C:\a\rust\rust\src\ci\citool\../../../build/citool\debug\deps;C:\a\rust\rust\src\ci\citool\../../../build/citool\debug;C:\Users\runneradmin\.rustup\toolchains\stable-aarch64-pc-windows-msvc\lib\rustlib\aarch64-pc-windows-msvc\lib;C:\Program Files\Git\clangarm64/bin;C:\Program Files\Git\usr\bin;C:\Users\runneradmin\bin;/c/a/rust/rust/ninja;/c/a/rust/rust/sccache;C:\Users\runneradmin/.cargo/bin;C:\aliyun-cli;C:\vcpkg;C:\Program Files (x86)\NSIS\;C:\Program Files\Mercurial\;C:\hostedtoolcache\windows\stack\3.3.1\x64;C:\mingw64\bin;C:\Program Files\dotnet;C:\Program Files\MySQL\MySQL Server 8.0\bin;C:\Program Files (x86)\R\R-4.4.2\bin\x64;C:\SeleniumWebDrivers\GeckoDriver;C:\SeleniumWebDrivers\EdgeDriver\;C:\SeleniumWebDrivers\ChromeDriver;C:\Program Files (x86)\sbt\bin;C:\Program Files (x86)\GitHub CLI;C:\Program Files\Git\bin;C:\Program Files (x86)\pipx_bin;C:\npm\prefix;C:\hostedtoolcache\windows\go\1.21.13\arm64\bin;C:\hostedtoolcache\windows\Python\3.9.13\x64\Scripts;C:\hostedtoolcache\windows\Python\3.9.13\x64;C:\hostedtoolcache\windows\Ruby\3.0.7\x64\bin;C:\Program Files\OpenSSL\bin;C:\tools\kotlinc\bin;C:\hostedtoolcache\windows\Java_Temurin-Hotspot_jdk\8.0.442-6\x64\bin;C:\Program Files (x86)\ImageMagick-7.1.1-Q16-HDRI;C:\Program Files\Microsoft SDKs\Azure\CLI2\wbin;C:\ProgramData\kind;C:\ProgramData\Chocolatey\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program Files\PowerShell\7\;C:\Program Files\Microsoft\Web Platform Installer\;C:\Program Files\Microsoft SQL Server\Client SDK\ODBC\170\Tools\Binn\;C:\Program Files\Microsoft SQL Server\150\Tools\Binn\;C:\Program Files\dotnet\;C:\Program Files\Microsoft SQL Server\130\DTS\Binn\;C:\Program Files\Microsoft SQL Server\140\DTS\Binn\;C:\Program Files\Microsoft SQL Server\150\DTS\Binn\;C:\Program Files\Microsoft SQL Server\160\DTS\Binn\;C:\Strawberry\c\bin;C:\Strawberry\perl\site\bin;C:\Strawberry\perl\bin;C:\ProgramData\chocolatey\lib\pulumi\tools\Pulumi\bin;C:\Program Files\CMake\bin;C:\ProgramData\chocolatey\lib\maven\apache-maven-3.9.9\bin;C:\Program Files\nodejs\;C:\Program Files\Git\cmd;C:\Program Files\Git\clangarm64\bin;C:\Program Files\Git\usr\bin;C:\Program Files\GitHub CLI\;c:\tools\php;C:\Program Files (x86)\sbt\bin;C:\Program Files\Amazon\AWSCLIV2\;C:\Program Files\Amazon\SessionManagerPlugin\bin\;C:\Program Files\Amazon\AWSSAMCLI\bin\;C:\Program Files\Microsoft SQL Server\130\Tools\Binn\;C:\Program Files\LLVM\bin;C:\Users\runneradmin\.dotnet\tools;C:\Users\runneradmin\AppData\Local\Microsoft\WindowsApps)
```
The fix for this is to use `cygpath` to convert from the Cygwin format back to native Windows format before adding to the `PATH`.

---
try-job: dist-aarch64-msvc
try-job: x86_64-msvc-1
try-job: x86_64-msvc-2
try-job: i686-msvc-1
try-job: i686-msvc-2
try-job: x86_64-mingw-1
try-job: x86_64-mingw-2
try-job: dist-i686-mingw

Author: bors