Docs build: depend on locally built nix executable and not installed one
(cherry picked from commit ca72e3e7e8f69526f028475a7a9b40812da1acdd)
===
includes changes from (because not doing so removes manpages):
Merge pull request #9976 from alois31/restore-manual-pages
Restore manual pages
(cherry picked from commit d3c1997127e0fc08576e842b2bfe046d8a28d2f4)
Change-Id: I685ff16163ac552a1754570c03c992c63a461d50