Fix reproduction instructions

Dependencies

• [2] 3LEGFJFP "Reproduce" action: Support flakes
• [3] JIJDYWPY Remove the Build menu from the top bar
• [*] J5UVLXOK * Start of a basic Catalyst web interface.

Change contents

• replacement in src/root/build.tt at line 542
  [2.261]→[2.261:375](∅→∅)
  − <span class="shell-prompt"># </span>nix build [% HTML.escape(eval.flake) %]:hydraJobs.[% HTML.escape(job.name) %]
  [2.261]

  [2.375]
  + <span class="shell-prompt"># </span>nix build [% HTML.escape(eval.flake) %]#hydraJobs.[% HTML.escape(job.name) %]