Giving `chezmoi apply` additional user defined variables

[StamesJames]

Is your feature request related to a problem? Please describe.

You are not able to have additional variables per chezmoi apply run. For example, if you have two different configs you want to switch between (for example for different themes) you would have to somehow change variables in the chezmoi config file for example

Describe the solution you'd like

Giving chezmoi apply the ability to read additional variables per run.
For example, chezmoi apply --vars some-vars-file.toml could read, in addition to all the other variables, the variables from some-vars-file.toml.

Describe alternatives you've considered

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Additional context

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[twpayne]

See also #4603.

[halostatue]

This feels…fragile.

It could be done with env vars, though: MY_ENV_VAR=foo chezmoi apply.

[twpayne]

I'm thinking of adding a command line flag --data like:

$ chezmoi apply --data '.foo="bar";.baz=1;qux=1.23'

This will allow you to add or override template data values for a single run.