[s390x][runners] Use consistent runner name across restarts

Currently, the runner name is taken from the docker container's
hostname.
This changes across restarts, causing the runner name to change across
restarts too.

This uses the host name to keep a consistent name.

ci/rootfs/s390x-self-hosted-builder/README.md

@@ -36,6 +36,7 @@ $ sudo systemctl daemon-reload
 $ sudo tee /etc/actions-runner-libbpf
 repo=<owner>/<name>
 access_token=<ghp_***>
+runner_name=<hostname>
 ```
 
 Access token should have the repo scope, consult

ci/rootfs/s390x-self-hosted-builder/fs/usr/bin/actions-runner

@@ -7,7 +7,7 @@
 #
 # - repo=<owner>/<name>
 # - access_token=<ghp_***>
-#
+# - runner_name=<hostname>
 
 set -e -u
 
@@ -34,6 +34,7 @@ registration_token=$(jq --raw-output .token "$token_file")
     --url "https://github.com/$repo" \
     --token "$registration_token" \
     --labels z15 \
+    --name "$runner_name" \
     --ephemeral
 
 # Run one job.