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.

Changed files
+1
pkgs
build-support
rust
build-rust-crate
+1
pkgs/build-support/rust/build-rust-crate/default.nix
···
metadata hasCrateBin crateBin verbose colors
extraRustcOpts buildTests codegenUnits;
};
installPhase = installCrate crateName metadata buildTests;
# depending on the test setting we are either producing something with bins
···
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