fix: ensure all post steps are executed (#1286)

This fixes an issue in the chain of post steps introduced
while updating the step-logger for composite actions.

It includes a test case so we make sure this does not happen
again.

pkg/runner/testdata/ensure-post-steps/action-post/post.js

@@ -0,0 +1,3 @@
+if (process.env["INPUT_FAIL"] === "true") {
+  process.exit(1);
+}