neomutt: make manual.txt not empty

Without elinks / w3m / lynx in the nativeBuildInputs, there are these
errors in the build:

LC_ALL=C w3m -dump -O UTF8 docs/manual.html > docs/manual.txt || \
LC_ALL=C lynx -dump -nolist -with_backspaces \
        -display_charset=us-ascii docs/manual.html > docs/manual.txt || \
LC_ALL=C elinks -dump -no-numbering -no-references \
        docs/manual.html | sed -e 's,\\001, ,g' > docs/manual.txt
/nix/store/xfbmj7sl2ikicym9x3yq7cms5qx1w39k-bash-4.4-p23/bin/bash: w3m: command not found
/nix/store/xfbmj7sl2ikicym9x3yq7cms5qx1w39k-bash-4.4-p23/bin/bash: line 1: lynx: command not found
/nix/store/xfbmj7sl2ikicym9x3yq7cms5qx1w39k-bash-4.4-p23/bin/bash: line 3: elinks: command not found