github/weekly: reduce test load

The weekly test runs out of current VM quota. Reduce the load slightly
by testing NUM_DOMAIN=7 only for one architecture. Pick ARM, since that
has the most complete proof stack.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

.github/workflows/weekly-clean.yml

@@ -18,9 +18,12 @@ jobs:
       fail-fast: false
       matrix:
         arch: [ARM, ARM_HYP, AARCH64, RISCV64, X64]
-        num_domains: ['1', '7', '']
+        num_domains: ['1', '']
         plat: ['']
         include:
+          - arch: ARM
+            num_domains: '7'
+
           - arch: ARM
             plat: exynos4
           - arch: ARM