fix: print hostname in error summary mail subject line

templates/error-summary.service.j2

@@ -4,7 +4,7 @@ Description=service daily Rosetta error summary
 [Service]
 Type=oneshot
 Restart=no
-ExecStart=/bin/bash -c '/operational_shared/software/error-summary.sh | /usr/bin/mail -s "Rosetta Error Summary from ${HOSTNAME}" "langzeitarchiv@slub-dresden.de"'
+ExecStart=/bin/bash -c '/operational_shared/software/error-summary.sh | /usr/bin/mail -s "Rosetta Error Summary from %H" "langzeitarchiv@slub-dresden.de"'
 User={{ vault_rosetta_user }}
 Group={{ vault_rosetta_group }}