Default to staging SFU for non-production builds

[awaitlink]

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A yarn ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

Reason for change

Currently, the staging builds (including the official ones for Public Username Testing (Staging Environment)) use the production SFU by default.

The production (or the test) SFU doesn't work (i.e. group calls are cancelled immediately) in the staging environment (because of different ZK parameters, I believe), requiring manually changing the SFU url via SignalDebug.setSfuUrl to the staging SFU.

Change

This PR changes the default SFU for builds to the staging SFU, overriding it with the production SFU only for production builds.

Other clients

With this PR, Desktop's behavior aligns with Signal Android and Signal iOS, both of which use the staging SFU by default in staging builds.

Testing

Successfully tried a group call (video and audio) between primary iOS physical device and the development Desktop for:

• Staging: Default build without any changes + iOS staging custom build 6.52.0.3.
• Production: Copying production.json to local-development.json and building + iOS beta 6.52.0.5.

[indutny-signal]

LGTM

[indutny-signal]

Thank you! This got merged and the PR will be closed when we do our next release!