when trying to use crane with a non local src (eg, not `./.`) we get the error
```
$ nix flake show
<...>
error: access to absolute path '/Cargo.toml' is forbidden in pure eval mode (use '--impure' to override)
(use '--show-trace' to show detailed location information)
```
To fix this, we quote the relative paths so to properly append them to the base path.
https://nixos.wiki/wiki/Nix_Expression_Language#Coercing_a_relative_path_with_interpolated_variables_to_an_absolute_path_.28for_imports.29
## Reproduction flake
```
{
description = "Build a cargo project without extra checks";
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
crane = {
url = "github:ipetkov/crane";
inputs.nixpkgs.follows = "nixpkgs";
};
external-crate-source = {
url = "github:ray-kast/empress";
flake = false;
};
flake-utils.url = "github:numtide/flake-utils";
};
outputs = { self, nixpkgs, crane, flake-utils, external-crate-source, ... }:
flake-utils.lib.eachDefaultSystem (system:
let
pkgs = nixpkgs.legacyPackages.${system};
test-crate = crane.lib.${system}.buildPackage {
src = external-crate-source.outPath;
};
in
{
packages.default = test-crate;
});
}
```
