textproc/scdoc: unbreak "make test" after r494629

Failed conversion of `2017-12-09 23:18:57'' using format `%F %T %z''
date: illegal time format
usage: date [-jnRu] [-d dst] [-r seconds|file] [-t west] [-v[+|-]val[ymwdHMS]]

[-I[date | hours | minutes | seconds]]
[-f fmt date | [[[[[cc]yy]mm]dd]HH]MM[.ss]] [+format]

Supports $SOURCE_DATE_EPOCH FAIL