Add tmux pkg to el8 and el9 images as user pkg

[aandvalenzuela]

Adding the package as requested in https://github.com/cms-sw/cms-docker/issues/237 To be merged when the new image rebuild is triggered

[cmsbuild]

A new Pull Request was created by @aandvalenzuela (Andrea Valenzuela) for branch master.

@iarspider, @smuzaffar, @aandvalenzuela, @cmsbuild can you please review it and eventually sign? Thanks. @antoniovilela, @rappoccio, @sextonkennedy you are the release manager for this. cms-bot commands are listed here

[smuzaffar]

ping