ci: provide ways to limit the number of "e2e tests" workflow runs

[tbouffard]

The e2e workflow currently runs 11 jobs. Since the GitHub organization allows a maximum of 20 concurrent jobs (with only 5 runners for macOS), it is essential to limit unnecessary runs.

This is particularly important when drafting a PR, as running these tests can often be deferred and re-enabled later. For example, temporarily disabling e2e tests during the drafting phase and re-enabling them later without committing configuration changes (e.g., removing or editing the workflow file) helps to significantly reduce redundant runs.

Implemented Improvements

• Concurrency Control: Automatically stop workflows for older commits when multiple commits are pushed in quick succession, ensuring that only the latest commit triggers the workflow.
• Skip on Demand: Enable skipping workflow runs by setting a specific label on the PR.

Notes

Closes #2172 Will be improved by #3196