Improve generated qt.toolchain.cmake usability

For platforms like Android and WebAssembly a separate toolchain file needs to be chainloaded from either the Android NDK or emscripten SDK.

When using a Qt online installed Qt's qt-cmake on the command line to configure a a project, configuration fails because the chainloaded toolchain file path from the CI is not present on the local machine.
The user is then forced to explicitly pass a -DQT_CHAINLOAD_TOOLCHAIN_FILE to their local SDK toolchain path. The Android qt.toolchain.cmake at least mentions the option, the WebAssembly one doesn't.

We should investigate if it's feasible to detect the NDK or emsdk env vars and derive the toolchain file paths from them.

I'm guessing when using Qt Creator, it's not a problem because Creator sets the relevant options.