cargo build-sbf: rust-toolchain.toml is ignored; pick rustc via --tools-version

cargo-build-sbf runs cargo +solana build (solana 1.18.x) or a generated +<toolchain> (agave 3.0.x, unless --no-rustup-override). rustup's +toolchain shorthand outranks rust-toolchain.toml: in a dir pinning 1.84, plain cargo gave 1.84.1 and cargo +stable gave 1.99.1 (rustup 1.29.2). So the file neither causes nor fixes 'requires rustc X, active is 1.75.0-dev'. The rustc comes from platform-tools. Per its release notes: v1.41-v1.42 = Rust 1.75.0, v1.43-v1.46 = 1.79.0, v1.47/v1.51 = 1.84.1, v1.54-v1.56 = 1.89.0, v1.57 = 1.95.0. --tools-version vX.Y exists in cargo-build-sbf 1.17.34, 1.18.26 and 3.0.6 (source-checked). agave 3.0.6 defaults to v1.51, 1.18.26 to v1.41.