cmdlib: bump default supermin VM memory request

Using mkfs.erofs has bumped the requirements for the amount of memory
we need here. While the memory usage was improved with the changes in
[1] and [2] we want to bump this here anyway to give ourselves some
wiggle room (especially because RHCOS is larger than FCOS).

[1] erofs/erofs-utils#13
[2] osbuild/osbuild#2020

Author: dustymabe

src/cmdlib.sh

@@ -767,7 +767,7 @@ EOF
 
     # There seems to be some false positives in shellcheck
     # https://github.com/koalaman/shellcheck/issues/2217
-    memory_default=2048
+    memory_default=3072
     # shellcheck disable=2031
     case $arch in
     # Power 8 page faults with 2G of memory in rpm-ostree