Make htmldocs separately

It calls `scripts/jobserver-exec`, which grabs as many jobserver tokens
as it can in order to calculate a `PARALLELISM` value to provide to the
command. Unfortunately, Sphinx grossly underutilizes the parallelism it
gets so slows down the build overall by denying tokens to other jobs.

PKGBUILD

@@ -78,7 +78,8 @@ prepare() {
 
 build() {
   cd $_srcname
-  _make htmldocs all
+  _make all
+  _make htmldocs
 }
 
 _package() {