fix(ci): pin third-party actions to full commit SHAs

The account's Actions policy requires a full-length SHA ref. A tag or branch ref is refused at
startup — `startup_failure`, no jobs, "this workflow graph cannot be shown" — so these workflows
could not run at all. This resolves each ref to the commit it currently points at and records the
ref in a trailing comment, e.g. `actions/checkout@<sha> # v4`.

`dtolnay/rust-toolchain` takes its toolchain from the ref itself, so those steps also gained an
explicit `with: toolchain:` input; without it, a SHA ref would silently lose the channel.

No behaviour is intended to change beyond the pins.

Author: hyperpolymath

.github/workflows/rhodibot.yml

@@ -34,7 +34,7 @@ jobs:
     timeout-minutes: 15
     steps:
       - name: Checkout
-        uses: actions/checkout@v7.0.1
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
         with:
           fetch-depth: 1
       - name: Rhodibot — detect drift (no mutations)