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/push-email-notify.yml

@@ -40,7 +40,7 @@ jobs:
     timeout-minutes: 5
     steps:
       - name: Send push notification email
-        uses: hyperpolymath/smtp-notify-action@v0.2.0 # NOSONAR — pin authority is actions.lock (sha1-ede1191ef6ff3ac02c4f4d9efdf837ee517e11d7)
+        uses: hyperpolymath/smtp-notify-action@ede1191ef6ff3ac02c4f4d9efdf837ee517e11d7  # NOSONAR — pin authority is actions.lock (sha1-ede1191ef6ff3ac02c4f4d9efdf837ee517e11d7)
         with:
           server_address: ${{ secrets.SMTP_HOST }}
           server_port: ${{ secrets.SMTP_PORT }}