picolisp: fix help functionality

Currently, trying to use help results in something like this:

```
: (help 'db)
========================================
w3m: Can't exec
=======================================
```

By including `w3m` as an input, the behaviour changes to:

```
: (help 'db)
========================================
(db 'sym 'cls ['hook] 'any ['sym 'any ..]) -> sym | NIL
Returns a database object of class cls, where the values for the sym arguments
correspond to the any arguments. If a matching object cannot be found, NIL is
returned. sym, cls and hook should specify a tree for cls or one of its
superclasses. See also aux, collect, request, fetch, init and step.

========================================
-> db
```

pkgs/development/interpreters/picolisp/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, jdk, makeWrapper }:
+{ stdenv, fetchurl, jdk, w3m, makeWrapper }:
 with stdenv.lib;
 
 stdenv.mkDerivation rec {
@@ -33,6 +33,7 @@ stdenv.mkDerivation rec {
 
 
     makeWrapper $out/bin/picolisp $out/bin/pil \
+      --prefix PATH : ${w3m}/bin \
       --add-flags "$out/lib/picolisp/lib.l" \
       --add-flags "@lib/misc.l" \
       --add-flags "@lib/btree.l" \