vm_image_util: Fix variable quoting to create correct OEM sysext file

The OEM sysext image file in the OEM partition had the version variable
name being part of the filename instead of the substituted version value
because of wrong quoting when the fixed string got replaced by ${…}.

build_library/vm_image_util.sh

@@ -531,7 +531,7 @@ install_oem_sysext() {
     local built_sysext_dir="${FLAGS_to}/${oem_sysext}-sysext"
     local built_sysext_filename="${oem_sysext}.raw"
     local built_sysext_path="${built_sysext_dir}/${built_sysext_filename}"
-    local version='${FLATCAR_VERSION}'
+    local version="${FLATCAR_VERSION}"
     local metapkg="coreos-base/${oem_sysext}"
     local build_sysext_flags=(
         --board="${BOARD}"