Pass --no-frozen-lockfile in pnpm dev mode to prevent CI failures (#23794) (CP: 25.1)

[vaadin-bot]

This PR cherry-picks changes from the original PR #23794 to branch 25.1.

Original PR description

Summary

• pnpm 10 auto-enables --frozen-lockfile when it detects CI environments
  (via ci-info, checking GITHUB_ACTIONS, CI, etc.)
• Flow never requests this: ciBuild defaults to false in dev mode
• When a user bumps vaadin.version and pushes, the stale pnpm-lock.yaml
  causes ERR_PNPM_LOCKFILE_CONFIG_MISMATCH in CI but works locally
• Fix: explicitly pass --no-frozen-lockfile when running pnpm install
  in dev mode (ciBuild==false), since Flow is resolving dependencies,
  not reproducing a locked build

Context

When ciBuild==true, Flow already passes --frozen-lockfile explicitly.
This change makes the non-CI path equally explicit, preventing pnpm from
silently inheriting a stricter mode based on environment detection.

Fixes #23530

[vaadin-bot]

This PR is eligible for auto-merging policy, so it has been approved automatically. If there are pending conditions, auto merge (with 'squash' method) has been enabled for this PR [Message is sent from bot]

[sonarqubecloud]

Quality Gate passed

Issues
136 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

 1 382 files  ±0   1 382 suites  ±0   1h 24m 19s ⏱️ - 1m 40s
 9 889 tests ±0   9 818 ✅ ±0  71 💤 ±0  0 ❌ ±0 
10 362 runs  ±0  10 282 ✅ ±0  80 💤 ±0  0 ❌ ±0 

Results for commit 845eaae. ± Comparison against base commit 0661df4.