runvm: Implement optional cache size for 'runvm' via env var

 - The current cache size for aarch64 during podman-related
operations is insufficient. We require a method to expand it
to enable the utilization of the cache feature by runvm. By
implementing this, we can conveniently adjust the cache size
as needed by passing it in the env var 'RUNVM_CACHE_SIZE'.

Signed-off-by: Renata Ravanelli <rravanel@redhat.com>

src/cmdlib.sh

@@ -601,9 +601,10 @@ runcompose_extensions() {
 # the cache disk. `runvm_with_cache_snapshot on` will set snapshotting to on.
 runvm_with_cache_snapshot() {
     local snapshot=$1; shift
+    local cache_size=${RUNVM_CACHE_SIZE:-16G}
     # "cache2" has an explicit label so we can find it in qemu easily
     if [ ! -f "${workdir}"/cache/cache2.qcow2 ]; then
-        qemu-img create -f qcow2 cache2.qcow2.tmp 16G
+        qemu-img create -f qcow2 cache2.qcow2.tmp "$cache_size"
         (
          # shellcheck source=src/libguestfish.sh
          source /usr/lib/coreos-assembler/libguestfish.sh