Properly detect debug build in slots (#3630)

`debug-assertions` can also be enabled in for release builds. This
introduces a new build-script that extracts the build type from the
`PROFILE` env variable and sets the `build_type` cfg.

substrate/core/consensus/slots/Cargo.toml

@@ -4,6 +4,7 @@ version = "2.0.0"
 authors = ["Parity Technologies <admin@parity.io>"]
 description = "Generic slots-based utilities for consensus"
 edition = "2018"
+build = "build.rs"
 
 [dependencies]
 codec = { package = "parity-scale-codec", version = "1.0.0" }