Builds in release then execs crates/target/release/qbz-slint directly instead of
via 'cargo run', so process monitors show it as 'qbz-slint' (a cargo-run child
inherits CARGO_* env and shows as 'cargo'), cleanly separate from the build
processes. Same release flags as slint-dev.sh.
cc
(cherry picked from commit 41269aabbe)