IBM Z: Run DFLTCC tests on the self-hosted builder

IBM Z: Run DFLTCC tests on the self-hosted builder

* Use the self-hosted builder instead of ubuntu-latest.
* Drop qemu-related settings from DFLTCC configurations.
* Install codecov only for the current user, since the self-hosted
  builder runs under a restricted non-root account.
* Use actions/checkout@v2 for configure checks, since for some reason
  actions/checkout@v1 cannot find git on the self-hosted builder.
* Update the testing section of the DFLTCC README.
* Add the infrastructure code for the self-hosted builder.