std.cache_hash: don't trust mtime granularity to be better than 1ms
I empirically observed mtime not changing when rapidly writing the same
file name within the same millisecond of wall clock time, despite the
mtime field having nanosecond precision.
I believe this fixes the CI test failures.
ce8b9c0c5cAndrew Kelley committed on 8/18/2020, 4:26:33 AMparent27cb23c