libstore/build.cc: more explicit error about form of output

Update "Upgrading Nix" documentation

doc: consistently refer to 'fixed-output' with a dash

Fix typo

General cleanup that makes it easier to search for the term.

This PR proposes two changes to the "Upgrading Nix" documentation:

* Besides updating `nixpkgs.nix`, we also update `nixpkgs.cacert`, so that the certificates are up-to-date as well.
* Add the instructions for multi-user mode on Linux.

Add unit tests

Update comment and enable the test

Add note about removal of trailing slashes in the doc comment of
baseNameOf and enabled the test.

Adjusted the doc comment for `dirOf` to reflect the implementation
behavior.

The function isn't being used anywhere so it seems safe to remove

No need to use `c_str()` in combination with `ASSERT_STREQ`.
It's possible to just use ASSERT_EQ on std::string