fix failing verify checksums for minio_archive

[jmelahman]

Apologies. I seemed to have introduced a regression in cc1c12a,

ERROR: /tmp/tmpss4hhm8j/external/cloud_archive/cloud_archive.bzl:28:13: An error occurred during the fetch of repository 'test_archive_p1_patch':
   Traceback (most recent call last):
	File "/tmp/tmpss4hhm8j/external/cloud_archive/cloud_archive.bzl", line 238, column 27, in _cloud_archive_impl
		cloud_archive_download(
	File "/tmp/tmpss4hhm8j/external/cloud_archive/cloud_archive.bzl", line 134, column 19, in cloud_archive_download
		cloud_download(repo_ctx, file_path, expected_sha256, provider, bucket, profile, file_version)
	File "/tmp/tmpss4hhm8j/external/cloud_archive/cloud_archive.bzl", line 221, column 22, in cloud_download
		validate_checksum(repo_ctx, file_path, downloaded_file_path, expected_sha256)
	File "/tmp/tmpss4hhm8j/external/cloud_archive/cloud_archive.bzl", line 28, column 13, in validate_checksum
		fail("Failed to verify checksum: {}".format(sha256_result.stderr))
Error in fail: Failed to verify checksum: sha256sum: local/bucket/data.tar.gz: No such file or directory
ERROR: Failed to verify checksum: sha256sum: local/bucket/data.tar.gz: No such file or directory

Confirmed the integration tests introduced in #12 pass with this applied