Original commit from NixOS#282226,
message:

Running CI locally is broken becauses the `-I` argument:

- Clobbers $NIX_PATH
- Is wrong for two reasons:
  - Has too many `..` elements, relative to the script's location
  - Isn't relative to the script's location (as with *.nix files),
    since shell scripts and POSIX in general interpret paths
    relative to the current working directory, not the canonical
    path of argv[0]
- Is inconsistent, since this script has symlinks pointing at it
  from different depths in the repository

There is no way to set this flag statically in a way that will work
everywhere.  The caller needs to use $NIX_PATH, or the script needs
to take the `-I` value as an argument.

This commit deletes the static `-I` flag.