minor: Explicitly disable the rust-analyzer extension in untrusted workspaces

[Veykril]

This is the default, but its always better to be explicit here + we can add a small note as to why.

[lnicola]

Hmm. Ideally, we would:

• disable build scripts, proc macros and check on save: this might be doable
• not respect paths from settings.json (does Code even load that from untrusted workspaces?): this is probably harder

[Veykril]

Ye that would be the goal #9224

[lnicola]

Okay, we can disable it now and fix it properly at some point in the future.

@bors r+

[bors]

📌 Commit 7fc0f89 has been approved by lnicola

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7fc0f89 with merge c468e39...

[bors]

☀️ Test successful - checks-actions
Approved by: lnicola
Pushing c468e39 to master...

[Veykril]

Note that it was disabled before as well already (it's the default with extensions that have an activation function afaik), this just adds a description as to why.