This reverts commit 1f5f599d0e
that bumped the zoneinfo version from 2020a to 2020b.

Apparently some libc are incompatible with the data format in 2020b.
At least our default libc, musl, fails to parse timezones from the
compiled 2020b zoneinfo files.

Release notes mention that deprecated "posixrules" feature is now
omitted from the default build. Possibly musl uses it?

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>