scripts: replace /bin/bash by /usr/bin/env bash

Test Plan: Set up environment locally; CI

X-Origin-Diff: phab/D759
GitOrigin-RevId: 01142b03dcfb44c1dcb42169cc9af187c1518107
diff --git a/scripts/gazelle.sh b/scripts/gazelle.sh
index eaffc80..0f37ae0 100755
--- a/scripts/gazelle.sh
+++ b/scripts/gazelle.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # gazelle.sh regenerates BUILD.bazel files for Go source files.
 set -euo pipefail