update-book2: optionally discover downloads even in local_genbook2

There used to be convenient download links for some of the ProGit book
translations.

In the transition to a Hugo-generated site, these links were lost for
all except the original English version, where the downloads were
hard-coded.

The reason is that in the Hugo world, we want to avoid network-heavy
roundtrips that would be necessary when using the `remote_genbook2`
function, and we use `local_genbook2` instead, where the information
about what downloadable assets there are (if any) is simply not there.

Let's add a way to discover the downloadable assets, by using Octokit,
but still use the local clone for the rest of the generation. The way to
enable this is by providing a `GITHUB_API_TOKEN` environment variable.
If that environment variable is not set, no dynamic discovery will be
performed.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

script/update-book2.rb

@@ -305,7 +305,11 @@ def local_genbook2(language_code, worktree_path)
       raise e
     end
     book.sha = `git -C "#{worktree_path}" rev-parse HEAD`.chomp
-    if language_code == 'en'
+    access_token = ENV.fetch("GITHUB_API_TOKEN", nil)
+    if access_token && Book.all_books[language_code]
+      @octokit = Octokit::Client.new(access_token:)
+      update_downloads(book, Book.all_books[language_code], @octokit)
+    elsif language_code == 'en'
       latest_tag = `git -C "#{worktree_path}" for-each-ref --format '%(refname:short)' --sort=-committerdate --count=1 refs/tags/`.chomp
       if latest_tag.empty?
         puts "No tag found in #{worktree_path}, trying to fetch tags"