[github] add pagerduty alert step to package manager test

[charliecruzan-stripe]

Reviewers

cc @stripe/developer-products

Summary

Adds a job that runs after all other jobs are completed- if any of those jobs failed, this job will run (which will page during business hours only).

You can see how this works via the test runs on the commits below. In first commit, the action does NOT run the pagerduty step since all needed jobs pass: https://github.com/stripe/stripe-cli/actions/runs/8929598608

In the second commit, the action does run since i forced the homebrew job fail: https://github.com/stripe/stripe-cli/actions/runs/8929626125

I've already added the secret to https://github.com/stripe/stripe-cli/settings/secrets/actions

[charliecruzan-stripe]

r? @fhats-stripe