keymgr zone key parsing broken with musl-libc

It seems that strptime() fails to parse our timestamps.

Reproducible on Alpine Linux.

alpine:/tmp/kasp# keymgr zone add example.com
alpine:/tmp/kasp# keymgr zone key generate example.com algo ecdsap256sha256 size 256
id d335fdaed59ea6eeb949e3d72b76aa090b16a88f keytag 39073
alpine:/tmp/kasp# keymgr zone key list example.com
Cannot retrieve zone from KASP (malformed config value)