Fix make vendor GOSUMDB
This fixes the intentionally undefine `GOSUMDB` variable when running `make
vendor` by setting `GOSUMDB` to a literal pair of quotes (""). This
corresponds to the value to be empty.
@@ -294,7 +294,7 @@ ${SHELLCHECK}:
curl -sSfL $$URL | tar xfJ - -C ${BUILD_BIN_PATH} --strip 1 shellcheck-$$VERSION/shellcheck && \
sha256sum ${SHELLCHECK} | grep -q $$SHA256SUM
-vendor: export GOSUMDB := ""
+vendor: export GOSUMDB :=
vendor:
$(GO) mod tidy
$(GO) mod vendor
