manual: build docs with dummy envs

this was previously used because the macOS docs build would otherwise
pull files out of the host nix store. or something. not sure about it

Change-Id: I76b51eac1ebc5de5f00e2e4be086dd8db3eeb8e6

doc/manual/meson.build

@@ -63,6 +63,7 @@ nix3_cli_json = custom_target(
   command : [ nix, '__dump-cli' ],
   capture : true,
   output : 'nix.json',
+  env : nix_env_for_docs,
 )
 
 generate_manual_deps = files(

doc/manual/src/command-ref/meson.build

@@ -62,4 +62,5 @@ conf_file_md = custom_target(
     experimental_features_shortlist_md,
   ],
   output : 'conf-file.md',
+  env : nix_env_for_docs,
 )