fix(git): Add a newline to the SSH private key (#34674)

Co-authored-by: Rhys Arkins <rhys@arkins.net>

lib/util/git/private-key.ts

@@ -87,7 +87,7 @@ class SSHKey extends PrivateKey {
     if (await this.hasPassphrase(keyFileName)) {
       throw new Error('SSH key must have an empty passhprase');
     }
-    await fs.outputFile(keyFileName, this.key);
+    await fs.outputFile(keyFileName, this.key.replace(/\n?$/, '\n'));
     process.on('exit', () => fs.removeSync(keyFileName));
     await fs.chmod(keyFileName, 0o600);
     // HACK: `git` calls `ssh-keygen -Y sign ...` internally for SSH-based