feat: allow excluding encrypted file in chezmoi step

[rubiin]

I want to suggest a new step

• Which tool is this about? Where is its repository?
• Which operating systems are supported by this tool?
• What should Topgrade do to figure out if the tool needs to be invoked?
• Which exact commands should Topgrade run?
• Does it have a --dry-run option? i.e., print what should be done and exit
• Does it need the user to confirm the execution? And does it provide a --yes
  option to skip this step?

I want to suggest some general feature

Topgrade should...

Allow an option to exclude the encrypted file

More information

More context can be found here twpayne/chezmoi#3747