Incorrect link in documentation for `external-builders` experimental feature

[Jayman2000]

Problem

The Experimental Features section of the manual has this to say about the external-builders experimental feature:

Refer to external-builders tracking issue for feature tracking.

The problem is that the “external-builders tracking issue” link doesn’t actually go to a page that is a tracking issue. It’s actually a link to the page that the reader was already on when they read that text.

Proposal

Fix the link so that it leads to a tracking issue about the external-builders experimental feature.

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

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