Cargo joins `build.rustflags` across config sources, so the base
`symbol-mangling-version=v0` and `tokio_unstable` flags still apply
after the newly added config.

With sccache disabled, `cargo build --release -p zed` on macOS: 
Before: zed binary 420677880 bytes
After: zed binary 375109048 bytes (-10.8%, -45.6MB)

After `strip -x` of both
(https://github.com/zed-industries/zed/pull/63429):
313879024 -> 307727432 bytes (-2.0%, -6.2MB)
The rest of the delta is the smaller symbol table. `drop_glue`
instantiations: 101778 -> 65041.

Release Notes:

- N/A