gdesktopappinfo: enable fast posix_spawn gspawn codepath
In order to use the new posix_spawn gspawn codepath, for more robust app launching when available memory is low, we need to meet some conditions. child_setup needs to be NULL for this optimization to work, so drop the internal child_setup that is used here. Replace it with a lightweight wrapper binary (gio-launch-desktop) that sets GIO_LAUNCHED_DESKTOP_FILE_PID before executing the app. Adjust PATH for gio tests so that it can execute the new binary from the build directory.
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