fix: ignore cache when reading user pjson

[mdonnalley]

Ignore the ProdOnlyCache when reading the user pjson because it could have been modified by a JIT install

Fixes issue with first execution of a JIT plugin failing with this error:

sf dev generate library                               
@salesforce/cli: Installing plugin dev@2.3.2... installed v2.3.2
 ›   ModuleLoadError: [MODULE_NOT_FOUND] import() failed to load /Users/peter.hale/.npm-global/lib/node_modules/@salesforce/cli/lib/commands/dev/generate/library.js: Cannot find module 
 ›   '/Users/peter.hale/.npm-global/lib/node_modules/@salesforce/cli/lib/commands/dev/generate/library.js' imported from /Users/peter.hale/.npm-global/lib/node_modules/@salesforce/cli/node_modules/@oclif/core/lib/module-loader.js
 ›   Code: MODULE_NOT_FOUND

Testing

1. Checkout branch, yarn build
2. cd to sf repo
3. yarn link @oclif/core
4. yarn oclif manifest
5. bin/run.js dev generate plugin

It should JIT install plugin-dev and successfully run the dev generate plugin command

[git2gus]

This issue has been linked to a new work item: W-16094608

[mshanemc]

QA notes:
✅ works as expected.
📓 I had to point the yarn/oclif from my CLI source repo to the "real" CLI I use from the installer