ci(rust): build, test and lint rhodibot

The rust.yml matrix was [robot-repo-automaton, shared-context, dashboard].
rhodibot - the crate implementing the GitHub App, and the one this branch
changes - was not in it, so it had never been compiled in CI. Adding it means
the gated commands (build --locked --all-targets, test --locked, clippy
--all-targets -- -D warnings) actually cover it.

Author: hyperpolymath

.github/workflows/rust.yml

@@ -28,7 +28,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        crate: [robot-repo-automaton, shared-context, dashboard]
+        # rhodibot was missing from this matrix, so the crate that ships the
+        # GitHub App had never been built, tested or linted in CI. Its
+        # verification was whatever a human remembered to run locally.
+        crate: [robot-repo-automaton, shared-context, dashboard, rhodibot]
     defaults:
       run:
         working-directory: ${{ matrix.crate }}