chore: not distribute externally on nightly build ios rc (#30142)

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## **Description**
Distribute externally only on experimental builds.
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2. What is the improvement/solution?
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## **Changelog**

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CHANGELOG entry:

## **Related issues**

Fixes:

## **Manual testing steps**

```gherkin
Feature: my feature name

  Scenario: user [verb for user action]
    Given [describe expected initial app state]

    When user [verb for user action]
    Then [describe expected outcome]
```

## **Screenshots/Recordings**

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### **After**

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## **Pre-merge author checklist**

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- [ ] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [ ] 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
- [ ] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

#### Performance checks (if applicable)

- [ ] I've tested on Android
  - Ideally on a mid-range device; emulator is acceptable
- [ ] I've tested with a power user scenario
- Use these [power-user
SRPs](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/edit-v2/401401446401?draftShareId=9d77e1e1-4bdc-4be1-9ebb-ccd916988d93)
to import wallets with many accounts and tokens
- [ ] I've instrumented key operations with Sentry traces for production
performance metrics
- See [`trace()`](/app/util/trace.ts) for usage and
[`addToken`](/app/components/Views/AddAsset/components/AddCustomToken/AddCustomToken.tsx#L274)
for an example

For performance guidelines and tooling, see the [Performance
Guide](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/400085549067/Performance+Guide+for+Engineers).

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- [ ] I've manually tested the PR (e.g. pull and build branch, run the
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- [ ] 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]
> **Low Risk**
> Low risk workflow-only change that just flips `distribute_external`
for the nightly iOS RC job; main impact is who receives TestFlight RC
builds.
> 
> **Overview**
> Nightly iOS RC builds triggered by `nightly-build.yml` are no longer
distributed externally to TestFlight by setting `distribute_external:
false` for the `ios-rc` job (experimental builds remain externally
distributed).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
7d31693. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: tommasini

.github/workflows/nightly-build.yml

@@ -43,7 +43,7 @@ jobs:
       source_branch: main
       environment: rc
       testflight_group: 'MetaMask BETA & Release Candidates'
-      distribute_external: true
+      distribute_external: false
     secrets: inherit
 
   # ── Android exp: ephemeral branch + build ──────────────────────────────