ci: re-try OBS distrotests
Often, the VM fails to boot (even twice in the row, as already handled by the test itself) which leads to false negative result.
These nightly tests fail far too often (at least on of the ~7 tests) and generate annoying notifications.
Adding yet another layer of retries should reduce the number of false negatives without diminishing the value of the test (since the real packaging issues are 100 % reproducible).
