ci: install clippy component before running it

`rust:1.94-trixie` ships rustc + cargo but not clippy — `rustup
component add clippy` pulls it in on the fly.

.forgejo/workflows/build-grid-rs.yaml

@@ -60,7 +60,9 @@ jobs:
           exit 1
 
       # Gate the image build on a clean test run so a push that regresses
-      # the scorer or breaks clippy never produces an image.
+      # the scorer or breaks clippy never produces an image. The stock
+      # `rust:1.94-trixie` image doesn't ship clippy, so add it before
+      # running.
       - name: cargo clippy + test
         run: |
           docker run --rm \
@@ -68,6 +70,7 @@ jobs:
             -w /src \
             rust:1.94-trixie \
             sh -euc '
+              rustup component add clippy
               cargo clippy --all-targets -- -D warnings
               cargo test
             '