Build PR prototypes on demand

[twstokes]

Description

Each push to a PR builds the prototypes and uploads them to Firebase App Distribution, though few PRs need them. This PR builds them only when someone asks: a Deploy Prototype Builds? button in the Buildkite build. Part of AINFRA-3348, which frees up CI agents. Pocket Casts iOS and Studio already work this way.

• The input sits in a group so that it blocks only the prototype step; the other checks run as before.
• The pipeline has publish_blocked_as_pending = false, so a build whose button is never clicked reports passed and blocked (success) to GitHub, not pending.
• The prototype step no longer sets notify: github_commit_status. Buildkite posted that status as pending as soon as the step waited for the input, which left a "Prototype Builds" check pending on every PR where nobody asked for prototypes. A failure still fails the overall build.
• "Prototype Builds" isn't a required check, so merges aren't affected.

Testing Steps

1. In this PR's Buildkite build, check that every step except the prototypes runs, that the build ends as passed and blocked, and that the PR shows no pending "Prototype Builds" check.
2. Click Deploy Prototype Builds?, confirm, and check that the prototypes build and are uploaded.

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