Nix pills links to the github repo are outdated

[kototama]

In the Nix Pills, the nixpkgs github repository code is linked multiple times for pedagogic reasons. For example when introducing the concept of a fixed point in paragraph 17.3. The problem is that the links are using the master branch. Since the nix pills have been written, the code in master has changed and the links do not point to the original concepts anymore. This makes it harder for newcomers to follow the Nix Pills introductions [1]

I would suggest pinning the links to the latest git commit (or multiple, if not possible) that still contain the concept. It would make the whole less confusing.

[1] which are already not always clear, for example try to make sense of this sentence:

For example, config.allowUnfree is an attribute that forbids building packages that have an unfree license by default.

It's not helpful to describe an attribute named allowUnfree with a negative term ("forbids"). Also grammatically it's not 100% clear to what the "by default" refers to. But ok that's a topic for a different issue...