Chartpress shouldn't modify version-controlled files

[manics]

Bug description

Many build tools avoid modifying version controlled source files, and instead write to a different output file. Chartpress modifies values.yaml in place, which means:

• It's necessary to have extra checks to prevent the modified file being committed: https://github.com/jupyterhub/chartpress/blob/main/.pre-commit-hooks.yaml
• Source control shows changes: BinderHub reported version is always .dirty binderhub#1621

Expected behaviour

It should be possible to run chartpress without git diff showing changes

Possible options

a. Rename values.yaml to something like values.yaml.in (or values.template.yaml, or similar), following the pattern of autotools which generates Makefile from Makefile.in, remove values.yaml from source control
b. Recursively copy the source directory to a temporary directory and do all work in that directory.

(a) can be made backwards compatible if we want by modifying values.yaml in-place if the input file isn't found, whereas (b) requires no change to repositories.