idris's wrapper should set IDRIS_CC to reduce the risk of accidentally using the wrong CC version

[catern]

Idris requires a C compiler to function. To find a C compiler, it first checks IDRIS_CC, as implemented here: https://github.com/idris-lang/Idris-dev/blob/master/src/IRTS/System.hs#L63 and then searches PATH.

The current wrapper for the "idris" compiler binary appends /nix/store/...-gcc-wrapper/bin to the PATH. This unfortunately makes idris quite fragile: When my PATH has some bad program named "cc" on it, such as some ancient compiler that I've installed for whatever reason, idris will break. It's also a fairly unnecessary impurity.

We should set IDRIS_CC in the wrapper instead, when it is not already set. This allows the user to still override the C compiler that is used if they want to, while significantly lowering the chance that things will be unexpectedly broken.