cmdlib: bump cache size to 30G

With us now building more in OSBuild in a single run [1] we need
a larger cache2.qcow2 to hold more pipeline outputs concurrently.

[1] coreos/fedora-coreos-pipeline#1055

src/cmdlib.sh

@@ -602,7 +602,7 @@ 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:-20G}
+    local cache_size=${RUNVM_CACHE_SIZE:-30G}
     # "cache2" has an explicit label so we can find it in qemu easily
     if [ ! -f "${workdir}"/cache/cache2.qcow2 ]; then
         qemu-img create -f qcow2 cache2.qcow2.tmp "$cache_size"