Fix support for .cargo/config

Before #300, our
`builtinz.toTOML` used to return a string representing the serialized
document which was later redirected into a file:

https://github.com/nix-community/naersk/pull/300/files#diff-b6b537316f2d29c8faf178a110366796811d1bc72f694262c7d2efad79aa984bL238

Over that merge request, this was changed to return a path, to make it
consistent with how nixpkgs' formatters work - but I haven't noticed one
place that still _read_ the file instead of returning a path (which was
fine before, since the code did `echo ... > ...`, but is not a correct
thing to do for `cp`).

Closes #305.

default.nix

@@ -31,7 +31,7 @@ let
         libb.findGitDependencies { inherit (config) cargolock gitAllRefs gitSubmodules; };
       cargoconfig =
         if builtinz.pathExists (toString config.root + "/.cargo/config")
-        then builtins.readFile (config.root + "/.cargo/config")
+        then (config.root + "/.cargo/config")
         else null;
       build = args: import ./build.nix (
         {

test/fast/cargo-config/default.nix

@@ -0,0 +1,6 @@
+{ naersk, ... }:
+
+naersk.buildPackage {
+  src = ./fixtures;
+  doCheck = true;
+}

test/fast/cargo-config/fixtures/.cargo/config

@@ -0,0 +1,2 @@
+[env]
+ENV_HELLO_WORLD="Hello, World!"

test/fast/cargo-config/fixtures/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "cargo-config"
+version = "0.1.0"
+edition = "2021"

test/fast/cargo-config/fixtures/src/main.rs

@@ -0,0 +1,11 @@
+fn main() {
+    println!("{}", env!("ENV_HELLO_WORLD"));
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test() {
+        assert_eq!("Hello, World!", env!("ENV_HELLO_WORLD"));
+    }
+}

test/fast/default.nix

@@ -1,4 +1,5 @@
 args: {
+  cargo-config = import ./cargo-config args;
   cargo-wildcard = import ./cargo-wildcard args;
   default-run = import ./default-run args;
   dummyfication = import ./dummyfication args;