build-rust-crate: dontStrip=!release

Without this PR, unlike `RUST_LIB_BACKTRACE=1 cargo run` you won't
get line numbers in backtraces from binaries built with:

```
nix build -f Cargo.nix --arg release false
```

This PR fixes that.

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -352,6 +352,7 @@ crate_: lib.makeOverridable
           metadata hasCrateBin crateBin verbose colors
           extraRustcOpts buildTests codegenUnits;
       };
+      dontStrip = !release;
       installPhase = installCrate crateName metadata buildTests;
 
       # depending on the test setting we are either producing something with bins