ci: Allow concurrent CI runs on main (#7031)

## Explanation

Fixes an issue where CI runs on main would be cancelled if there is a
newer commit, despite `cancel-in-progress` being set to `false`.

## References

MetaMask/metamask-mobile#21893

## 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](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [ ] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Update workflow concurrency group to use commit SHA on main,
differentiating runs on main while keeping existing cancel-in-progress
logic.
> 
> - **CI / GitHub Actions**
> - **Concurrency**: Update `concurrency.group` in
`.github/workflows/main.yml` to `${{ github.workflow }}-${{ github.ref
== 'refs/heads/main' && github.sha || github.ref }}` to use `github.sha`
on `main` and `github.ref` otherwise.
> - `cancel-in-progress` remains `${{ !contains(github.ref,
'refs/heads/main') }}`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ea221a5. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Mrtenz

.github/workflows/main.yml

@@ -6,7 +6,7 @@ on:
   pull_request:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.ref == 'refs/heads/main' && github.sha || github.ref }}
   cancel-in-progress: ${{ !contains(github.ref, 'refs/heads/main') }}
 
 jobs: