pom: Cache tzdata before entering capability mode

[emaste]

Summary

Pom uses localtime() so needs tzdata available.

PR:             278574
Reported by:    Tatsuki Makino

[ivy]

looks right to me, although i didn't check the maths.

old behaviour (TZ is ignored):

% TZ=America/New_York pom -p -d 2025-07-27
6
% TZ=Pacific/Kiritimati pom -p -d 2025-07-27
6

new behaviour:

% TZ=America/New_York pom -p -d 2025-07-27
6
% TZ=Pacific/Kiritimati pom -p -d 2025-07-27
3