We force using Emscripten 1.38.47 as there is a regression in 1.38.48
and 1.39.x which makes the generation of our .wasm file fail.
Up until now we used the desktop BCL (compiled with desktop Mono)
for both desktop and Android, but that does not work for WebAssembly
(and we suspect Android issues related to it).
So we now build the Android and WebAssembly BCLs in the relevant
containers, which are later copied in the templates directory to
include on export. We already have the desktop BCL from the regular
build.
This saves us from pulling gtk2 and gtk3 as weak deps of
perl and git or whatnot... And we don't need to pull in all
the documentation on Ubuntu.
Also ensures that we properly run `dnf clean all` after each
installation.
