Only add private SSH keys and clone submodules in the original repo

[jmartinesp]

Content

• Disable adding private SSH keys and cloning submodules in the CI if the repo isn't this one.
• Also disable Danger jobs since the fallback GH token won't let any forks write comments in PRs, so the issues can't be reported.

Motivation and context

Most steps on PRs from forks are now broken.

Tests

Fork status: #3223
There should be no change for this PR.

Checklist

• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR

[github-actions]

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[bmarty]

Thanks!