Solana: reuse Agave binary, do not rebuild the workspace

Solana: reuse Agave binary, do not rebuild the workspace

• Install looks for agave-validator in /opt/solana/<env>/bin and /var/tmp/rpcnode-agave-src-*/{bin,target/release} and copies it if cargo already produced the file.
• Source build is cargo build --release --bin agave-validator only. cargo-install-all.sh is not used (it compiles the whole workspace: reqwest, rustls, …).
• Retry does not rm -rf an existing clone.
• Identity uses /opt/solana/<env>/bin/solana-keygen (was empty optPath → keygen not found after a successful cargo build).