nixos/doc/writing-nixos-tests: Clarify working directory

Co-authored-by: christian-burger <[e-mail]>

nixos/doc/manual/development/writing-nixos-tests.section.md

···
 You can run a test with attribute name `mytest` in `nixos/tests/all-tests.nix` by invoking:
 
 ```shell
+cd /my/git/clone/of/nixpkgs
 nix-build -A nixosTests.mytest
 ```
 

nixos/doc/manual/from_md/development/writing-nixos-tests.section.xml

···
         in <literal>nixos/tests/all-tests.nix</literal> by invoking:
       </para>
       <programlisting>
+cd /my/git/clone/of/nixpkgs
 nix-build -A nixosTests.mytest
 </programlisting>
     </section>