ci.yml: Set RUSTDOCFLAGS to statically link doctests

Prior to Rust 1.89, these tests were just skipped when cross-compiling. Now, they are actually compiled and ran. Unfortunately, doctests don't use the normal `RUSTFLAGS` environment variable, so we also need to set `RUSTDOCFLAGS` or else the resulting dynamically linked executable will fail to run on a non-Android host. Upstream change: https://github.com/rust-lang/cargo/pull/15462