install_sct: Install libffi from conda-forge, rather than defaults

[joshuacwnewton]

Description

The defaults version of libffi performs some (presumably) backwards-compatible linking between versions of libffi. However, this isn't actually compatible as assumed; instead, this results in the following errors:

Check if figure can be opened with PyQt.............libGL error: MESA-LOADER: failed to open iris: /lib/x86_64-linux-gnu/libLLVM-11.so.1: undefined symbol: ffi_type_sint32, version LIBFFI_BASE_7.0 (search paths /usr/lib/x86_64-linux-gnu/dri:\$${ORIGIN}/dri:/usr/lib/dri)
libGL error: failed to load driver: iris
libGL error: MESA-LOADER: failed to open iris: /lib/x86_64-linux-gnu/libLLVM-11.so.1: undefined symbol: ffi_type_sint32, version LIBFFI_BASE_7.0 (search paths /usr/lib/x86_64-linux-gnu/dri:\$${ORIGIN}/dri:/usr/lib/dri)
libGL error: failed to load driver: iris
libGL error: MESA-LOADER: failed to open swrast: /lib/x86_64-linux-gnu/libLLVM-11.so.1: undefined symbol: ffi_type_sint32, version LIBFFI_BASE_7.0 (search paths /usr/lib/x86_64-linux-gnu/dri:\$${ORIGIN}/dri:/usr/lib/dri)
libGL error: failed to load driver: swrast

So, we install the conda-forge version of libffi, which does not (and will not) have this issue, as confirmed by the conda-forge feedstock maintainer.

Linked issues

Fixes #3927.

[mguaypaq]

Thanks for the follow-up with upstream!