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

[dpaoliello]

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.

[rustbot]

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