fix(.npmrc): clean up .npmrc file in the repo after releasing

Issue: "Failed to replace env in config: ${NPM_TOKEN}" after release #2
solution：semantic-release/semantic-release#974 (comment)

src/index.js

@@ -60,6 +60,17 @@ const release = async () => {
     ...(handleDryRunOption()),
   });
 
+  // Clean up `.npmrc` file in the repo after releasing
+  {
+    const {stdout, stderr} = await exec(`rm -f .npmrc`, {
+      cwd: path.resolve(__dirname)
+    });
+    core.debug(stdout);
+    if (stderr) {
+      core.debug(stderr);
+    }
+  }
+
   if (!result) {
     core.debug('No release published.');
     return;