fix(ci/tangled): set TZDIR so tzinfo gem finds zoneinfo in /nix/store
First Spindle run failed at config.rb:74 (Middleman 'Time.zone =' /
'set :timezone' line) with TZInfo::DataSources::ZoneinfoDirectoryNotFound.
tzinfo's default search path is ['/usr/share/zoneinfo', '/etc/zoneinfo'];
nixery's tzdata package puts zoneinfo under /nix/store/<hash>-tzdata-XXXX/
share/zoneinfo. Locate it via find + export TZDIR before the build step.