build: try to use synth for the nightly run again (#2220)

The GitHub Action experiment did not work out for a few reasons:

1. It would appear that a PR created by the `github-actions` token will not trigger github actions (CI in this case) to actually run.  This is apparently by design.
2. The next logical step would be to put the `yoshi-automation` personal access token in the secrets for the repository, and use that.  That's a security can of worms I'd prefer to leave unopened.
3. We ran into some issues with OSPO on approving CLA bot to approve PRs from the `github-actions` user account. 

Effectively, relying on GitHub Actions to submit a PR for our purposes is probably a bad idea.  *Instead*, I want to do the same thing from our existing infrastructure, in kokoro.  This will give me the same keys I need.

The only gotcha here is that I am submitting my own PRs from this script, and *do not* want to rely on the autosynth behavior of submitting the change PR for me.  I have a hunch that the branch gymnastics I am doing will lead to no changes getting submitted by autosynth, but we will have to wait and see.

synth.py

@@ -22,8 +22,10 @@
 common_templates = gcp.CommonTemplates()
 templates = common_templates.node_library()
 s.copy(templates, excludes=[
-  ".github/CONTRIBUTING.md", 
+  ".github/CONTRIBUTING.md",
   "README.md",
   ".eslintignore",
   ".prettierignore"
 ])
+subprocess.run(['npm', 'install'])
+subprocess.run(['npm', 'run', 'submit-prs'])