feat: use shallow clone in prepare and emit-build-metadata jobs (#29807)

## **Description**

The `prepare` and `emit-build-metadata` jobs in `build.yml` use
`fetch-depth: 0` (full git history clone), which takes ~1m 10s each
(median, n=21). Neither job performs any git history operations:

- `prepare`: reads `builds.yml` via `fs.readFileSync` and runs
`validate-build-config.js` (zero git commands)
- `emit-build-metadata`: runs `git rev-parse HEAD` (works on shallow
clones) and `get-build-metadata.sh` which only reads files (zero git
commands)

Removing `fetch-depth: 0` defaults to `fetch-depth: 1` (shallow clone),
reducing checkout from ~70s to ~10-15s per job.

**Expected saving:** ~1m 50s - 2m 10s per pipeline run across both jobs.
Also eliminates a tail-risk outlier where `emit-build-metadata` checkout
took 6m 45s.

Part of MCWP-574 (pipeline optimization series: PR 3 of 4).

## **Changelog**

CHANGELOG entry: null

## **Related issues**

Fixes: MCWP-574

## **Manual testing steps**

```gherkin
Feature: Shallow clone in build pipeline

  Scenario: prepare job completes with shallow clone
    Given a production build is triggered via workflow_dispatch on build.yml
      And skip_version_bump is true

    When the prepare job runs
    Then the actions/checkout step completes in ~10-15s (down from ~70s)
      And the prepare job outputs are identical to a full-clone run

  Scenario: emit-build-metadata job completes with shallow clone
    Given a production build completes successfully

    When the emit-build-metadata job runs
    Then the actions/checkout step completes in ~10-15s
      And built_commit_sha, semantic_version, and version codes are output correctly

  Scenario: version-bump commit ref works with shallow clone
    Given a production build is triggered with skip_version_bump false

    When the prepare job checks out the version-bump commit hash
    Then actions/checkout fetches the specific commit successfully
      And the prepare job completes without errors
```

## **Screenshots/Recordings**

### **Before**

Not applicable (CI pipeline change, no UI).

### **After**

Not applicable (CI pipeline change, no UI).

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [ ] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

Author: Qbandev

.github/workflows/build.yml

@@ -117,7 +117,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          fetch-depth: 0
           ref: ${{ !inputs.skip_version_bump && needs.update-build-version.outputs.commit-hash || (inputs.source_branch || github.ref_name) }}
       - name: Setup Node.js
         uses: actions/setup-node@v4
@@ -528,7 +527,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          fetch-depth: 0
           ref: ${{ needs.prepare.outputs.checkout_ref_for_setup }}
 
       - name: Setup Node.js