Since tzcode 2020b (upstream commit 6ba6f2117b95), zic defaults to -b
'slim'. This has been synced in glibc 2.40.

Using this option causes the 32-bit data entries to be omited, however
some packages like python-dateutil still relies on it. Therefore
explicitly build with -b 'fat' for now.

Closes: #1084111

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>