fix(ci): decouple prepare job from action-checkout-and-setup

[cryptodev-2s]

Explanation

The upgrade to action-checkout-and-setup from v2 to v3 (#8047) broke the prepare job. v3 skips the git checkout on cache hits, causing yarn workspaces list to produce empty output, which makes fromJson fail with "empty input".

This PR takes the long-term approach: replace the composite action entirely with a plain actions/checkout + corepack enable. The prepare job only lists workspace package names — it doesn't need caching, yarn install, multiple Node versions, or setup-node. This makes it:

• Faster — no dependency installation, runs in seconds
• Simpler — 2 setup steps instead of a heavy composite action
• Resilient — future changes to action-checkout-and-setup (v4, v5, etc.) can't break it again

See #8053 for the minimal quick-fix alternative.

References

• Related to chore(deps): bump MetaMask/action-checkout-and-setup from 2 to 3 #8047
• Alternative to fix(ci): remove matrix from prepare job to fix fromJson empty input #8053

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Low risk workflow-only change that simplifies the prepare job, but CI could fail if corepack/yarn/jq aren’t available or if checkout behavior differs from the composite action.

Overview
Fixes CI flakiness in lint-build-test.yml by decoupling the prepare job from MetaMask/action-checkout-and-setup@v3, which could skip checkout on cache hits.

prepare now just runs actions/checkout@v5 + corepack enable before computing child-workspace-package-names via yarn workspaces list, removing the prior Node-version matrix and caching/setup complexity for this job.

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