[CI] Fix issues with checking PRs created against repository forks

[mwu-tow]

Pull Request Description

This PR fixes several issues that were appearing when running CI jobs on PRs created against the repository forks:

• electron-builder on Windows and macOS will properly recognize that the secrets are missing and will not attempt to sign the artifacts;
• similarly, fixed the S3 library tests;
• test reporter step will be now skipped, as it does not support forks.

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  TypeScript,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• Unit tests have been written where possible.