Change hey args, as entry point is now necessary

[hash-d]

The hey Containerfile introduced in https://github.com/skupperproject/skupper/pull/1652/files#diff-a49f0e686902bad1c7d315c3d0961ac9686e01493982c4f0c3f98c560c409352 defines a CMD, but not an ENTRYPOINT.

The HTTP performance test is not reflecting that change, and failing with the error below:

Error: container create failed: executable file `-z` not found in $PATH: No such file or directory

This PR addresses that error.