tree-wide: stop assigning colon prefixed timezone to $TZ

glibc (and also musl, though we do not officially support it yet)
silently ignores colon prefix in $TZ. Let's always not prefix the
timezone.

tzset(3) states:
> A nonempty value of TZ can be one of two formats, either of which can
> be preceded by a colon which is ignored.

Addresses https://github.com/systemd/systemd/pull/38876#discussion_r2384347594.

src/shared/calendarspec.c

@@ -1436,10 +1436,7 @@ int calendar_spec_next_usec(const CalendarSpec *spec, usec_t usec, usec_t *ret_n
 
         SAVE_TIMEZONE;
 
-        /* tzset(3) says $TZ should be prefixed with ":" if we reference timezone files */
-        const char *colon_tz = strjoina(":", spec->timezone);
-
-        r = RET_NERRNO(setenv("TZ", colon_tz, 1));
+        r = RET_NERRNO(setenv("TZ", spec->timezone, /* overwrite = */ true));
         if (r < 0)
                 return r;