Previously idris used the C compiler from PATH for the C backend, which
means that the results and whether it even succeeds can vary between
systems (e.g. if a Nix-built Idris was used on a super old Linux system,
the cc installed there might not even work for Idris' C).

To make this more predictable, this commit sets the IDRIS_CC env var,
which Idris will prefer over searching in PATH, to a Nix-provided gcc
executable, given that it is not already set, so it's still possible to
override.

Idris: Wrap with IDRIS_CC