ci: Enforce lock file when retrieving dependencies

[Mr-Pepe]

For reproducibility, we should always use the lock file when
retrieving dependencies and fail otherwise.

[Mr-Pepe]

Current dependencies on/for this PR:

• main
  • PR ci: Auto-approve on PR review #45
    • PR chore: Check devtools_options.yaml into Git #42
      • PR chore: Add dependabot configuration #43
        • PR ci: Cache Flutter and pub packages #44
          • PR ci: Cache .dart_tool directory #46
            • PR ci: Enforce lock file when retrieving dependencies #47 👈
              • PR ci: Prevent duplicate concurrent CI runs #48

This stack of pull requests is managed by Graphite.

[Mr-Pepe]

LGTM

[Mr-Pepe]

Merge activity

• Jan 9, 12:39 PM: @Mr-Pepe started a stack merge that includes this pull request via Graphite.
• Jan 9, 12:40 PM: Graphite rebased this pull request as part of a merge.
• Jan 9, 12:41 PM: @Mr-Pepe merged this pull request with Graphite.