hscolour-bootstrap: allow setting RTS options for the hscolour binary we use to generate our documentation

Other tools like hsc2hs or haddock allows this, too, and some tools explicitly
set memory limits on these tools during the build (i.e. llvm-general-pure) by
exposing a GHCRTS environment variable. See [1] for a concrete example of this
issue.

[1] https://github.com/NixOS/nixpkgs/issues/2939