fix e2e-infra-cleanup workflow

[cbodonnell]

What this PR does / why we need it:

fixes e2e-infra-cleanup workflow by adding a timestamp to the terraform workspace name so that the creation time can be retrieved more reliably. example of it failing silently https://github.com/replicatedhq/kots/actions/runs/7019202274/job/19096137049

would resolve issues like:

date: invalid date ‘\nWarning: No outputs found\n\nThe state file either has no outputs defined, or all the defined outputs are\nempty. Please define an output in your configuration with the `output`\nkeyword and run `terraform refresh` for it to become available. If you are\nusing interpolation, please verify the interpolated value is not empty. You\ncan use the `terraform console` command to assist.’

and

Error: Output "completion_timestamp" not found

The output variable requested could not be found in the state file. If you
recently added this to your configuration, be sure to run `terraform apply`,
since the state won't be updated with new output variables until that command
is run.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Steps to reproduce

Does this PR introduce a user-facing change?

Does this PR require documentation?