Fix for building updated docs during Homebrew install

I noticed that the docs weren't building when trying to `brew install fish --HEAD` due to Homebrew installing inside it's 'superenv' during installation - `which sed` returns a shiv while writing the shebang to `lexicon_filter` which breaks the docs build. 

I submitted a simple fix, but the consensus was that we needed something more robust, so this allows brew to override the `LEXICON_SED_PATH` variable during make. See Homebrew/homebrew#32315. I've got an updated brew formula ready once this is merged.