The most common case for validation will be that the wrapper jars are unchanged
from a previous workflow run. In this case, we cache the validated wrapper
checksums to minimise the work required on a subsequent run.
Fixes#172
The checksum values for most wrapper versions are hard-coded into the
action. These known checksum values are first used for validation: only
if none of the known values work do we download checksums.
Previously, we blindly downloaded all of the checksum values in this
case: we now only download the checksums for versions that are not in
our "known" set.
Fixes#171
