change cygpath to mixed

[matt-sm]

cygpath -w will print \ eg. c:\Users\msmith.

When this is used to calculate EXT_BUILD_ROOT, and EXT_BUILD_ROOT is used as a suffix for other paths, it can lead to a mix of \ amd / which can cause configure_make build failures. eg.

unsafe srcdir value: 'C:\tmp\2b4dntvp\execroot\__main__/external/

cygpath comes preinstalled in msys2, therefore is always used to calculate pwd.

The fallback, pwd -W will print / eg. c:/Users/msmith.

This MR changes the Windows pwd() to use the cygpath -m "mixed" mode that matches pwd -W.

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[matt-sm]

It appears meson prefers c:\..\ paths.

Program win_flex found: YES (C:\tmp\766s67rb\execroot\rules_foreign_cc_examples\bazel-out\x64_windows-opt-exec-2B5CBBC6-ST-2324745666c0\bin\external\winflexbison\copy_winflexbison\winflexbison\win_flex.EXE)

when c:\..\.. in PATH vs

Program win_flex found: NO

when c:/../ in PATH

I noticed those tool paths are in the build script but I'm not sure how they're being inserted.

[jsharpe]

Closing in favour of #1204