The visual_test_runner binary (added in #45259) has `required-features =
["visual-tests"]`, so it doesn't get built in release builds. However,
`cargo bundle` was trying to bundle all binaries defined in the zed
crate's Cargo.toml, causing it to fail when visual_test_runner doesn't
exist.

Adding `--bin zed` tells cargo bundle to only bundle the main zed
binary.

**Note:** The `cli` binary is unaffected by this change - it's a
separate package (`crates/cli`) that's built separately via `cargo build
--package cli` and manually copied into the app bundle afterward (line
337). It was never part of `cargo bundle`'s scope.

Fixes the nightly build failure:
https://github.com/zed-industries/zed/actions/runs/20740971838

Release Notes:

- N/A