internal: add `git clean -df` to grafeas/containeranalysis protections
internal: add `git clean -df` to grafeas/containeranalysis protections

Change-Id: I639dfe60a1a41c913787941e9905cc4fcd034ce8
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/42313
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go