//tests:stamp_test inputs become stale too quickly

I often test all of `rules_pkg` using something like `bazelisk test //...`.

When I leave my desk for the day and come back the following day, I sometimes see `//tests:stamp_test` mysteriously fail after making some changes. This is due to the timestamps no longer being valid:

```
Traceback (most recent call last):
File "/private/var/tmp/_bazel_apsaltis/67ca5e5a723760c2131afe9d288ecae5/sandbox/darwin-sandbox/9/execroot/rules_pkg/bazel-out/darwin-fastbuild/bin/tests/stamp_test.runfiles/rules_pkg/tests/stamp_test.py", line 104, in test_not_epoch_times_tar
self.assertTarFilesAreAlmostNew('stamped_tar.tar')
File "/private/var/tmp/_bazel_apsaltis/67ca5e5a723760c2131afe9d288ecae5/sandbox/darwin-sandbox/9/execroot/rules_pkg/bazel-out/darwin-fastbuild/bin/tests/stamp_test.runfiles/rules_pkg/tests/stamp_test.py", line 82, in assertTarFilesAreAlmostNew
self.check_mtime(info.mtime, file_path, info.name)
File "/private/var/tmp/_bazel_apsaltis/67ca5e5a723760c2131afe9d288ecae5/sandbox/darwin-sandbox/9/execroot/rules_pkg/bazel-out/darwin-fastbuild/bin/tests/stamp_test.runfiles/rules_pkg/tests/stamp_test.py", line 63, in check_mtime
self.fail(
AssertionError: Archive /private/var/tmp/_bazel_apsaltis/67ca5e5a723760c2131afe9d288ecae5/sandbox/darwin-sandbox/9/execroot/rules_pkg/bazel-out/darwin-fastbuild/bin/tests/stamp_test.runfiles/rules_pkg/tests/stamped_tar.tar contains file . with mtime:1639538959, expected:1639586337 +/- 10000
```

It would be nice if the relevant artifacts could be rebuilt when they have expired in the cache. I don't know how implement this, though.

This problem is never seen in CI, since it builds everything from scratch all the time.