(maint) Add beaker-hostgenerator version

This is a follow-up to c05b7c7. PDK does not add environment variables
to the Gemfile unless a version is specified in .sync.yml, so this
commit adds a version for beaker-hostgenerator.

Author: mhashizume

.sync.yml

@@ -20,6 +20,7 @@ Gemfile:
         version: '~> 1.0'
       - gem: beaker-hostgenerator
         from_env: BEAKER_HOSTGENERATOR_VERSION
+        version: '~> 2.0'
       - gem: beaker-rspec
         from_env: BEAKER_RSPEC_VERSION
       # Prevent beaker-puppet from being installed on Ruby > 3.1 until beaker-puppet supports newer Rubies (PA-6136)