Confirm to RFC RFC0043: Reproducible builds

[robdimsdale]

To confirm to RFC0043 this buildpack should ensure that builds are reproducible. Specifically it should not include a built_at metadata field. In the tests that leverage this field to assert layer reuse, we should instead compare layer SHA values across rebuilds.

See also the tracking issue: paketo-buildpacks/rfcs#165.

[robdimsdale]

As part of this, we should also add an integration test for layer reuse (see https://github.com/paketo-buildpacks/cpython/blob/main/integration/layer_reuse_test.go for an example).

[ForestEckhardt]

@robdimsdale As far as I can tell this buildpack does not actually reuse a layer currently. It has a cache layer that is revived each build but the build happens regardless of the metadata found in the previous build. I think that means a layer reuse test is not necessary. If I am missing something feel free to point it out.