flake: Avoid setting HELIX_RUNTIME in devShell

The runtime directory should be correctly set without the need to set
HELIX_RUNTIME manually because we check for a runtime directory within
CARGO_MANIFEST_DIR.

This change also filters the runtime directory out of the source file
set passed to buildRustPackage since the runtime directory is not needed
at compilation time.

default.nix

@@ -16,6 +16,7 @@
     ./screenshot.png
     ./book
     ./docs
+    ./runtime
     ./flake.lock
     (fs.fileFilter (file: lib.strings.hasInfix ".git" file.name) ./.)
     (fs.fileFilter (file: file.hasExt "svg") ./.)

flake.nix

@@ -72,7 +72,6 @@
             ++ (lib.optional stdenv.isLinux lldb)
             ++ (lib.optional stdenv.isDarwin darwin.apple_sdk.frameworks.CoreFoundation);
           shellHook = ''
-            export HELIX_RUNTIME="$PWD/runtime"
             export RUST_BACKTRACE="1"
             export RUSTFLAGS="''${RUSTFLAGS:-""} ${rustFlagsEnv}"
           '';