radiant: build netradiant as non-PIE binary on Linux to workaround a mime-type bug

radiant: build netradiant as non-PIE binary on Linux to workaround a mime-type bug

PIE executables were wrongly recognized as shared libraries on Linux
because of a mistake in the mime-type library.
Because of that it was not possible to launch such executables from
the file browser.

While the problem is fixed upstream, most distributions still ship
with a faulty mime-type library and may do for a long time.

See: https://gitlab.freedesktop.org/xdg/shared-mime-info/-/issues/11