common: Fix typo with docker image loading.

The typo shouldn't have had any effect -- the kokoro instance should
be invoking this code exactly once on a fresh device each time, so we'd
expect the load to occur exactly once which is what is currently
happening.

BUG=None
TEST=None

Change-Id: I5d274ca5489984898ce4a8855a6afde54a488e87
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/containers/cros-container-guest-tools/+/1915090
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
Commit-Queue: David Riley <davidriley@chromium.org>
Tested-by: David Riley <davidriley@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/kokoro/common_build.sh b/kokoro/common_build.sh
index 27829ea..ed6b829 100644
--- a/kokoro/common_build.sh
+++ b/kokoro/common_build.sh
@@ -26,7 +26,7 @@
         local base_image="buildmesa_${dist}"
         local base_image_tarball="${KOKORO_GFILE_DIR}"/"${base_image}".tar.xz
 
-        if [[ -z $(docker images -q $"{base_image}" 2> /dev/null) ]]; then
+        if [[ -z $(docker images -q "${base_image}" 2> /dev/null) ]]; then
             docker load -i "${base_image_tarball}"
         fi