docs: add missing PYTEST_JIRA_USERNAME to Jira integration section

Add the missing PYTEST_JIRA_USERNAME environment variable to the Jira
integration section in RUNNING_TESTS.md. The utilities/jira.py module
requires all three variables (PYTEST_JIRA_URL, PYTEST_JIRA_TOKEN, and
PYTEST_JIRA_USERNAME) but the documentation only listed two.

Assisted-by: Claude <noreply@anthropic.com>
Signed-off-by: rnetser <rnetser@redhat.com>

Author: rnetser

docs/RUNNING_TESTS.md

@@ -180,6 +180,7 @@ To use jira plugin, you need to set the following environment variables:
 ```bash
 export PYTEST_JIRA_URL=<url>
 export PYTEST_JIRA_TOKEN=<your_token>
+export PYTEST_JIRA_USERNAME=<email>    # email associated with the Jira account
 ```
 
 ## Additional options