Switch to the zlib flate2 backend

The default `miniz_oxide` flate2 backend has poor performance in debug/under QEMU:
rust-lang/flate2-rs#297

Ideally we'd use the fastest `zlib-ng` backend, however it fails to cross-compile:
rust-lang/libz-sys#93

As such we have to use the next best alternate backend, which is `zlib`.

This makes the `flate2` usage in this repo consistent with `libherokubuildpack`,
Python CNB, PHP CNB etc.

buildpacks/ruby/Cargo.toml

@@ -9,7 +9,7 @@ workspace = true
 [dependencies]
 clap = { version = "4", default-features = false, features = ["derive", "error-context", "help", "std", "usage"] }
 commons = { path = "../../commons" }
-flate2 = "1"
+flate2 = { version = "1", default-features = false, features = ["zlib"] }
 fs-err = "2"
 fun_run = { version = "0.1", features = ["which_problem"] }
 glob = "0.3"