Add some simple e2e tests for executing CLI plugins

To help with this add a bad plugin which produces invalid metadata and arrange
for it to be built in the e2e container.

Signed-off-by: Ian Campbell <ijc@docker.com>

scripts/build/plugins

@@ -8,7 +8,7 @@ set -eu -o pipefail
 source ./scripts/build/.variables
 
 mkdir -p "build/plugins-${GOOS}-${GOARCH}"
-for p in cli-plugins/examples/* ; do
+for p in cli-plugins/examples/* "$@" ; do
     [ -d "$p" ] || continue
 
     n=$(basename "$p")