copr-be: boost the s390x builders (from max 12 to max 18)

Nourhane @ IBM confirmed this should be OK, considering we spend about
the same cloud budget as before (more machines means we still process
the same amount of tasks, but faster).

inventory/group_vars/copr_aws

@@ -41,7 +41,7 @@ builders:
     x86_64: [20, 4, 20]
   ibm_cloud_us_east:
     # s390x is on-demand
-    s390x: [12, 6, 6]
+    s390x: [18, 6, 6]
   ibm_cloud_tokyo:
     # tokyo is evacuated
     s390x: [0, 6, 6]

roles/copr/backend/templates/copr-be.conf.j2

@@ -55,7 +55,7 @@ builds_max_workers_arch=
     ppc64le={{ max_ppc64le_workers }}
 
 # Do not let one user waste the whole architecture capacity
-build_max_workers_arch_per_owner=s390x=8
+build_max_workers_arch_per_owner=s390x=12
 
 # Maximum number of concurrently running tasks per project owner.
 {% if env == 'production' %}