oci_tarball: set mtime to 2000-01-01 #908

Job	Run time
matrix-prep-bazelversion	3s
test-auth	3m 4s
test (ubuntu-latest, 6.2.0, ., true)	3m 25s
test (ubuntu-latest, 6.2.0, ., false)	1m 48s
test (ubuntu-latest, 6.2.0, e2e/custom_registry, false)	41s
test (ubuntu-latest, 6.2.0, e2e/wasm, false)	36s
test (ubuntu-latest, 6.2.0, e2e/smoke, true)	20s
test (ubuntu-latest, 6.2.0, e2e/smoke, false)	16s
test (ubuntu-latest, 6.2.0, e2e/crane_as_registry, true)	17s
test (ubuntu-latest, 6.2.0, e2e/crane_as_registry, false)	19s
test (ubuntu-latest, 5.3.2, ., false)	3m 10s
test (ubuntu-latest, 5.3.2, e2e/custom_registry, false)	33s
test (ubuntu-latest, 5.3.2, e2e/wasm, false)	1m 2s
test (ubuntu-latest, 5.3.2, e2e/smoke, false)	33s
test (ubuntu-latest, 5.3.2, e2e/crane_as_registry, false)	32s
	16m 39s

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