Ignore following non-constant source

jonknapp · jonknapp · commit 9a3bd01ee469 · 2017-09-04T10:09:47.000-04:00
diff --git a/bin/private/asdf-exec b/bin/private/asdf-exec
@@ -34,6 +34,7 @@ for version in "${versions[@]}"; do
         export ASDF_INSTALL_VERSION=$version
         export ASDF_INSTALL_PATH=$install_path
 
+        # shellcheck source=/dev/null
         source "${plugin_path}/bin/exec-env"
 
         # unset everything, we don't want to pollute
