Fix bug in the generic-builder.txt file of Pill 8

[modulitos]

Under this section: https://nixos.org/guides/nix-pills/generic-builders.html#idm140737320224432

There is a loop over buildInputs like this:

for p in $buildInputs; do
  export PATH=$p/bin${PATH:+:}$PATH
done

but buildInputs, which is defined in autotools.nix, is empty, resulting in an empty PATH. This PR fixes that typo so that it iterates over $baseInputs instead.