Mark first-party generated files with .gen; preserve third-party conventions

src/capability-language/tool-cli.ts

@@ -437,6 +437,7 @@ const main = async () => {
       for (const [executable, args] of [
         ["corepack", ["yarn", "plugin", "import", toolchain.nixifyPluginUrl]],
         ["corepack", ["yarn", "config", "set", "generateDefaultNix", "false"]],
+        ["corepack", ["yarn", "config", "set", "nixExprPath", "yarn-project.gen.nix"]],
         ["corepack", ["yarn", "config", "set", "individualNixPackaging", "true"]],
         ["corepack", ["yarn", "install"]],
       ] as const) {
@@ -447,10 +448,10 @@ const main = async () => {
           );
       }
       try {
-        await readFile(path.join(cwd, "yarn-project.nix"));
+        await readFile(path.join(cwd, "yarn-project.gen.nix"));
       } catch {
         throw new Error(
-          "Nixify did not generate yarn-project.nix. It skips repositories under the OS temporary directory; use an ordinary workspace checkout and retry installation.",
+          "Nixify did not generate yarn-project.gen.nix. It skips repositories under the OS temporary directory; use an ordinary workspace checkout and retry installation.",
         );
       }
       await snapshotCommit(cwd);