worker/integration: update run to not require a specific cwd
worker/integration: update run to not require a specific cwd

as pointed out by @pivotal-jamie-klassen [1], the previous way that we
referenced the project root was dependant on what the current working
directory (cwd) was.

by retrieving the directory of $0 (the name used to call the script,
which would include that path to it when `cwd` is not `.`) with a
combination of `realpath`, we can point to the root of the concourse
repository regardless of where we call the script from.

[1]: https://github.com/concourse/concourse/pull/4988#discussion_r364844776

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse