nix develop: Set $out to a writable location

Currently $out (and other environment variables corresponding to outputs) point to a non-existant, non-writable location in the Nix store. Thus commands like make install don't work. Some packages work around this by doing stuff like:

          shellHook =
            ''
              export prefix=$(pwd)/inst
              configureFlags+=" --prefix=$prefix"
              PKG_CONFIG_PATH=$prefix/lib/pkgconfig:$PKG_CONFIG_PATH
              PATH=$prefix/bin:$PATH
            ";

We should make $out point to a writable location by default. The question is what it should be. It should be persistent across nix develop invocations and per-project, so maybe something like $(pwd)/outputs/$outputName.

tweag / nix-ux

nix develop: Set $out to a writable location #8