Fix CI: use cargo build-sbf directly instead of build script

The build-sbf.sh script uses --install-only which isn't supported in
v2.0.25's cargo-build-sbf. In CI, Solana is already on PATH so we
can call cargo build-sbf directly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: MagicalTux

.github/workflows/verifiable-build.yml

@@ -23,7 +23,7 @@ jobs:
         run: solana-keygen new --no-bip39-passphrase
 
       - name: Build program
-        run: ./scripts/build-sbf.sh
+        run: cargo build-sbf --workspace
 
       - name: Start test validator
         run: |