[rush] Update ci.yml to cache installed dependencies

[microsoftly]

Summary

There are no clear examples of how to best cache installed dependencies using rushjs. It would be a boon to both rushstack and developers to be able to check the framework repo's ci.yml to see how to implement a primitive for CI operations.

Repro steps

Repro steps not needed

Details

See summary

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QnA unneeded.

[iclanton]

It'd probably be pretty straightforward to cache the common/temp/node_modules and common/temp/*/node_modules (for subspaces) folders using the GitHub Actions cache. Dependency install happens pretty quickly in our CI, so we haven't investigated that yet, but if you want to put together a proposed change, we'd probably take it.

Right now we've mostly been investigating using the GitHub Actions cache for builds (see related issue actions/toolkit#1760).

[dmichon-msft]

Since we install via pnpm by default, this first becomes a question of the best way to cache package installation for pnpm.

Guidance using this as a reference is to cache the pnpm-store: https://github.com/pnpm/action-setup