Speed up CI by caching rustup toolchain and using cargo-binstall

Cache ~/.rustup so the toolchain isn't reinstalled every run, and
install cargo-make/cargo-audit via cargo-binstall instead of
compiling them from source each time.

.forgejo/workflows/ci.yml

@@ -17,6 +17,7 @@ jobs:
           path: |
             ~/.cargo/registry
             ~/.cargo/bin
+            ~/.rustup
             target
           key: ${{ runner.os }}-cargo-${{ hashFiles('Cargo.lock') }}
           restore-keys: |
@@ -24,13 +25,20 @@ jobs:
 
       - name: Install Rust toolchain
         run: |
+          if [ ! -x "$HOME/.cargo/bin/rustc" ]; then
             curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --profile minimal --component clippy --component rustfmt
+          fi
           echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
 
+      - name: Install cargo-binstall
+        run: |
+          command -v cargo-binstall >/dev/null 2>&1 || \
+            curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/cargo-bins/cargo-binstall/main/install-from-binstall-release.sh | bash
+
       - name: Install cargo-make and cargo-audit
         run: |
           for crate in cargo-make cargo-audit; do
-            command -v "$crate" >/dev/null 2>&1 || cargo install "$crate"
+            command -v "$crate" >/dev/null 2>&1 || cargo binstall -y "$crate"
           done
 
       - name: Run CI checks