build: correctly propagate changes to Lix source to manual

This adds the nix executable as an input argument to the custom targets
that build the manual (HTML, and the man pages), so that changes to
things like src/nix/nix.md cause the correct rebuilds

Change-Id: Iffaa2c14acbfc721caef7b8cae9b53ecd365f26d

doc/manual/meson.build

@@ -110,6 +110,7 @@ manual = custom_target(
     builtins_md,
     builtin_constants_md,
     rl_next_generated,
+    nix,
   ],
   output : [
     'manual',
@@ -186,6 +187,7 @@ foreach command : nix_nested_manpages
       ],
       input : [
         manual_md,
+        nix,
       ],
       output : command[0] + '-' + page + '.1',
       install : true,
@@ -298,6 +300,7 @@ foreach page : nix3_manpages
     input : [
       'render-manpage.sh',
       manual_md,
+      nix,
     ],
     output : page + '.1',
     install : true,
@@ -341,6 +344,7 @@ foreach entry : nix_manpages
       'render-manpage.sh',
       manual_md,
       entry.get(3, []),
+      nix,
     ],
     output : '@0@.@1@'.format(entry[0], entry[1]),
     install : true,