MSRV violation

See tpo/core/arti!2231 (comment 3043017)

error: unsupported output in build script of `derive-deftly-macros v0.13.0`: `cargo::rustc-check-cfg=cfg(derive_deftly_dprint)`
Found a `cargo::key=value` build directive which is reserved for future use.
Either change the directive to `cargo:key=value` syntax (note the single `:`) or upgrade your version of Rust.

1.56 ignores this completely.