PERF: Switch plugin:install_all_official to clone plugins concurrently (#24511)

Why this change?

`plugin:install_all_official` is quite slow at the moment taking roughly
1 minute and 51 seconds on my machine. Since most of the time is spent
waiting on the network, we can actually speed up the Rake task
significantly by executing the cloning concurrently. With a 8 cores
machine, cloning all plugins will only take 15 seconds.

What does this change do?

This change wraps the `git clone` operation in the
`plugin:install_all_official` Rake task in a `Concurrent::Promise` which
basically runs the `git clone` operation in a Thread. The `--quiet`
option has also been added to `git clone` since running stuff
concurrently messes up the output. That could be fixed but it has been
determined to be not worth it since the output from `git clone` is
meaningless to us.

lib/tasks/plugin.rake

@@ -8,14 +8,18 @@ task "plugin:install_all_official" do
 
   STDERR.puts "Allowing write to all repos!" if ENV["GIT_WRITE"]
 
+  promises = []
+  failures = []
+
   Plugin::Metadata::OFFICIAL_PLUGINS.each do |name|
     next if skip.include? name
+
     repo = "https://github.com/discourse/#{name}"
     dir = repo.split("/").last
     path = File.expand_path("plugins/" + dir)
 
     if Dir.exist? path
-      STDERR.puts "Skipping #{dir} cause it already exists!"
+      STDOUT.puts "Skipping #{dir} cause it already exists!"
       next
     end
 
@@ -24,16 +28,24 @@ task "plugin:install_all_official" do
       repo += ".git"
     end
 
-    attempts = 0
-    begin
-      attempts += 1
-      system("git clone #{repo} #{path}", exception: true)
+    promises << Concurrent::Promise.execute do
+      attempts ||= 1
+      STDOUT.puts("Cloning '#{repo}' to '#{path}'...")
+      system("git clone --quiet #{repo} #{path}", exception: true)
     rescue StandardError
-      abort("Failed to clone #{repo}") if attempts >= 3
-      STDERR.puts "Failed to clone #{repo}... trying again..."
+      if attempts == 3
+        failures << repo
+        abort
+      end
+
+      STDOUT.puts "Failed to clone #{repo}... trying again..."
+      attempts += 1
       retry
     end
   end
+
+  Concurrent::Promise.zip(*promises).value!
+  failures.each { |repo| STDOUT.puts "Failed to clone #{repo}" } if failures.present?
 end
 
 desc "install plugin"