Marginally improve convert/ffmpeg calls performance with posix-spawn (#7346)

Gemfile

@@ -60,6 +60,7 @@ gem 'nsa', '~> 0.2'
 gem 'oj', '~> 3.5'
 gem 'ostatus2', '~> 2.0'
 gem 'ox', '~> 2.9'
+gem 'posix-spawn', '~> 0.3'
 gem 'pundit', '~> 1.1'
 gem 'premailer-rails'
 gem 'rack-attack', '~> 5.2'

Gemfile.lock

@@ -381,6 +381,7 @@ GEM
     pghero (2.1.0)
       activerecord
     pkg-config (1.3.0)
+    posix-spawn (0.3.13)
     powerpack (0.1.1)
     premailer (1.11.1)
       addressable
@@ -705,6 +706,7 @@ DEPENDENCIES
   pg (~> 1.0)
   pghero (~> 2.1)
   pkg-config (~> 1.3)
+  posix-spawn (~> 0.3)
   premailer-rails
   private_address_check (~> 0.4.1)
   pry-byebug (~> 3.6)