Maintain a symlink to the rust toolchain in the project root

This helps with IntelliJ's setup, rather than finding the toolchain in
the Nix store.

.gitignore

@@ -1,6 +1,7 @@
 .direnv/
 .idea/
 /target
+.rust-toolchain
 *.act
 *.bmp
 *.jpg

flake.nix

@@ -47,6 +47,17 @@
           darwin.apple_sdk.frameworks.SystemConfiguration
         ];
         RUST_BACKTRACE = 1;
+
+        # Copy rust-toolchain to project directory for easy use in IntelliJ
+        shellHook = ''
+          if [ -L ./.rust-toolchain ] && [ "$(readlink ./.rust-toolchain)" = "${rust-toolchain}" ]; then
+            echo "Rust toolchain symlink is already correct."
+          else
+            rm -f ./.rust-toolchain
+            ln -s ${rust-toolchain} ./.rust-toolchain
+            echo "Rust toolchain symlink updated."
+          fi
+        '';
       };
     });
 }