build: install html manual without using install_subdir

In Meson, `install_subdir` is meant to be used with directories in the source
directory. When using it to install the HTML manual, we provide it with a path
under the build directory.

We should instead specify an install directory for the HTML manual as part of
the custom target that builds it.

What we do currently isn't broken, just semantically incorrect. Changing it does
get rid of the following deprecation warning, though:

``
Project [...] uses feature deprecated since '0.60.0': install_subdir with empty directory. It worked by accident and is buggy. Use install_emptydir instead.
``

Change-Id: I259583b7bdff8ecbb3b342653d70dc5f034c7fad

doc/manual/meson.build

@@ -126,20 +126,18 @@ manual = custom_target(
     'manual',
     'markdown',
   ],
+  install_dir : [
+    datadir / 'doc/nix',
+    false,
+  ],
   depfile : 'manual.d',
   env : {
     'RUST_LOG': 'info',
     'MDBOOK_SUBSTITUTE_SEARCH': meson.current_build_dir() / 'src',
   },
 )
-manual_html = manual[0]
 manual_md = manual[1]
 
-install_subdir(
-  manual_html.full_path(),
-  install_dir : datadir / 'doc/nix',
-)
-
 nix_nested_manpages = [
   [ 'nix-env',
     [