Fix `rapids-date-string` (#68)
As it turns out, using the `YYMMDD` format in `rapids-date-string` for
wheels broke some things.
This PR adjusts `rapids-date-string` to generate both `YYMMDD` strings
and "seconds since epoch" strings for wheels.
Since the new wheel scripts all call `rapida-date-string` without any
arguments, the default argument is set to "second".
The default argument should be removed once `rapids-env-update` is
deprecated and the individual repositories have been updated to call
`rapids-date-string` in this new format.
`rapids-env-update` has also been updated to pass the `day` argument
since all conda workflows currently get their `RAPIDS_DATE_STRING` value
through that script.
To avoid having to pass arguments from the calling scripts in the
future, we could create two new `gha-tools` scripts (e.g.
`rapids-wheel-date-string` and `rapids-conda-date-string`) that call a
"private" script (`_rapids-date-string`) with the appropriate arguments.
