Add caching to GitHub Actions workflow

Cache Cargo registry/binaries and Solana CLI to significantly reduce
CI build times. The Cargo cache is keyed by Cargo.lock hash, and the
Solana cache is keyed by version (2.3.11).

This should reduce the ~6 minute Anchor installation time and avoid
re-downloading Solana CLI on every run.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mikemaccana

.github/workflows/test.yml

@@ -9,16 +9,32 @@ on:
 jobs:
   run-arcium-build:
     runs-on: ubuntu-latest
-    # TODO: Add caching for Rust, Solana CLI, Anchor, and Arcium installations
-    # to reduce CI build times. Consider using actions/cache for:
-    # - ~/.cargo (Rust toolchain and Anchor)
-    # - ~/.local/share/solana (Solana CLI)
     steps:
       - uses: actions/checkout@v3
 
       - name: Install libudev
         run: sudo apt-get update && sudo apt-get install -y libudev-dev
 
+      - name: Cache Cargo registry and installed binaries
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-
+
+      - name: Cache Solana CLI
+        uses: actions/cache@v3
+        with:
+          path: ~/.local/share/solana/install
+          key: ${{ runner.os }}-solana-2.3.11
+          restore-keys: |
+            ${{ runner.os }}-solana-
+
       - name: Install Rust
         run: |
           curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y