cmd/cue/cmd: use ParseInt with 64 bit size in cmd_after testdata

The current milliseconds since Epoch is out of bound for a 32-bit int. Use
int64 explicitly here. This change allows the test case to run successfully
on 32-bit platforms.

Closes #4021 as merged as of commit 13375f1c.

Note from Daniel: we didn't catch this in CI as we only run 32-bit tests
with `go test -short ./...`, and tests like this one which use sleeps
are marked as long. So we got a bit unlucky.

If this happens again in the future, we can teach our CI to skip the use
of `-short` when testing on the main branch.

Signed-off-by: Hoang Nguyen <folliekazetani@protonmail.com>
Change-Id: I2c679a6490469606f8be10706127015bea332b19
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1220033
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>