ci: Allow concurrent runs on main (#37395)

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## **Description**

Currently the `main.yml` workflow only runs once per combination of
workflow and branch.

Usually if a new commit is pushed to a branch, the CI run on the
previous commit is no longer relevant. However that's not true on main.
Getting a full CI run for every commit on main is critical for
investigating CI failures that somehow make their way to main (e.g.
flaky test failures).

The concurrency group ID has been updated to use the SHA as the ID on
just the main branch, ensuring new commits don't cancel CI workflows for
previous commits.

Based on mobile one:
MetaMask/metamask-mobile#21893

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## **Related issues**

Fixes:

## **Manual testing steps**

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## **Screenshots/Recordings**

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### **Before**

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### **After**

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## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/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
- [ ] 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-extension/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.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Update GitHub Actions concurrency group to use commit SHA on `main`,
enabling concurrent CI runs per commit on the main branch.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0ed9612. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: jvbriones

.github/workflows/main.yml

@@ -22,7 +22,7 @@ on:
     - cron: '0 2-6 * * *'
 
 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') || contains(github.ref, 'refs/heads/stable')) }}
 
 env: