Fix numpy benchmark suite running

Author: timfel

mx.graalpython/mx_graalpython_python_benchmarks.py

@@ -627,7 +627,7 @@ def _vmRun(self, vm, workdir, command, benchmarks, bmSuiteArgs):
             mx.run(["git", "config", "user.email", "[email protected]"], cwd=npdir)
             mx.run(["git", "config", "user.name", "YourName"], cwd=npdir)
             mx.run(["git", "commit", "--allow-empty", "-m", "init"], cwd=npdir)
-            mx.run(["git", "branch", self.VERSION], cwd=npdir)
+            mx.run(["git", "branch", f"v{self.VERSION}"], cwd=npdir)
             mx.run(["git", "branch", "main"], cwd=npdir, nonZeroIsFatal=False)
             mx.run(["git", "branch", "master"], cwd=npdir, nonZeroIsFatal=False)