chore: update `openvm build` nightly version
For guest programs with MSRV 1.87+, it is convenient for `cargo openvm
build` to use a newer Rust toolchain. We chose `2025-08-02` since that
is when the last stable Rust 1.90.0 branched from master: https://releases.rs/docs/1.90.0/
Note that users can always override the toolchain version with env var
`OPENVM_RUST_TOOLCHAIN`.