Bump omnibus from ac881b7 to c5c10bf in /omnibus

Bumps [omnibus](https://github.com/chef/omnibus) from `ac881b7` to `c5c10bf`.
- [Release notes](https://github.com/chef/omnibus/releases)
- [Commits](chef/omnibus@ac881b7...c5c10bf)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

omnibus/Gemfile.lock

@@ -9,10 +9,10 @@ GIT
 
 GIT
   remote: https://github.com/chef/omnibus.git
-  revision: ac881b7f810218323eeb4f71af4a8a4db75c1c92
+  revision: c5c10bff2b34bcf530a7cb215eb819ac6a3913c6
   branch: master
   specs:
-    omnibus (6.1.4)
+    omnibus (6.1.9)
       aws-sdk-s3 (~> 1)
       chef-cleanroom (~> 1.0)
       chef-sugar-ng (>= 3.3)