Always use POSIX paths for generated CMake files (#65493)

The Windows plugin CMake generation had code to ensure that the paths
written to it used POSIX separators, but the Linux version didn't; that
meant that plugin updates run on Windows machines would corrupt the
generated (but checked in) Linux CMake file.

This change shares that code so that both will use POSIX paths
regardless of what OS they are generated on.

Fixes https://github.com/flutter/flutter/issues/64591