tests: Unset SOURCE_DATE_EPOCH

[smcv]

Some distributions set this during build in order to have reproducible
builds from the same source code: for example, Debian uses the date
from debian/changelog.

However, some of our tests assume that ostree commit will result in
a commit with the current date/time, and SOURCE_DATE_EPOCH breaks that
assumption. Unset it for our build-time tests.

Resolves: #2405

[openshift-ci]

Hi @smcv. Thanks for your PR.

I'm waiting for a ostreedev member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[smcv]

I'm trying this with a Debian package build, but I think it'll work.

[lucab]

/ok-to-test

[smcv]

A modified Debian package build, reverting my packaging-level workaround for #2405 and adding this patch instead, was successful.