fix: long-running cluster access script needs to run from local machine

[tommartensen]

... instead of via curl.
That is because it is sourcing lib.sh from the same directory, which is obviously not available if only the script is downloaded.

Example error:

$ curl -L https://raw.githubusercontent.com/stackrox/stackrox/master/scripts/release-tools/setup-central-access.sh | bash -s -- long-fake-load-4.2.2-rc.1
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   813  100   813    0     0   2944      0 --:--:-- --:--:-- --:--:--  3000
bash: line 7: ./lib.sh: No such file or directory

This updates the Slack notification.