build.sh: freeze grub2 to fix PXE + UEFI tests

FCOS build started to fail after grub2.2-06-123.fc40 update. We are freezing the previous version to fix it. You can have more information about it in the following issue: coreos#3815

build.sh

@@ -43,6 +43,15 @@ install_rpms() {
 
     frozendeps=""
 
+    # freeze grub2 for https://github.com/coreos/coreos-assembler/issues/3815
+    case "${arch}" in
+          x86_64) frozendeps=$(echo grub2-{common,efi-x64,pc,pc-modules,tools,tools-extra,tools-minimal}-2.06-121.fc40);;
+        aarch64) frozendeps=$(echo grub2-{common,efi-aa64,tools,tools-extra,tools-minimal}-2.06-121.fc40);;
+        ppc64le) frozendeps=$(echo grub2-{common,efi-aa64,tools,tools-extra,tools-minimal}-2.06-121.fc40);; 
+        *) ;;
+    esac
+
+
     # First, a general update; this is best practice.  We also hit an issue recently
     # where qemu implicitly depended on an updated libusbx but didn't have a versioned
     # requires https://bugzilla.redhat.com/show_bug.cgi?id=1625641