Feature Request: pnpm is not supported for `pkg-manager`

[dbeal-eth]

For our official CircleCI Docker Convenience Image support policy, please see CircleCI docs.

This policy outlines the release, update, and deprecation policy for CircleCI Docker Convenience Images.

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Describe the Feature Request
Can't use pnpm with this cimg because it is not supported (apparently)

Is your feature request related to a particular problem?
No

How will this feature request benefit CircleCI jobs using this image?
This image will be compatible with projects using pnpm as their package manager.

Describe the solution you would like to see
pnpm is supported value for pkg-manager for this cimg, and pnpm is used as the package manager when that setting is used.

Describe alternatives you have considered
I haven't considered any alternatives.

[cglango]

This would be covered by #403