main/musl: backport fix for TZ parsing logic
This fixes usage of TZ value which do not contain a slash character with
musl libc. Such as TZ=CET. I doubt that there will be a new musl release
before Alpine's 3.15 feature freeze and I would really like this fix to
be included in the next release as it fixes real bugs in real software
(see !19695 (closed) for example).