Run go fmt over packages, not entire directory

Otherwise the now persistent Go mod cache is checked as well, and builds
will fail if any dependency is not gofmt'd.

Fixes #484.

build/package/scripts/build-go.sh

@@ -71,7 +71,7 @@ echo GOMODCACHE="$GOMODCACHE"
 df -h "$ROOT_DIR"
 
 echo "Checking format ..."
-unformatted=$(gofmt -l .)
+unformatted=$(go fmt $(go list ./...))
 if [ -n "${unformatted}" ]; then
   echo "Unformatted files:"
   echo "${unformatted}"