/bin/bash does not exist on certain systems

Bazel and rules_scala refer to `/bin/bash` which does not exist on certain systems. In particular on [NixOS](https://nixos.org/) where the issue was discovered. Replacing `/bin/bash` by `/usr/bin/env bash` resolves the issue.

On NixOS this problem is usually circumvented by the corresponding package description for bazel patching these paths. However, rules_scala downloads the `java_stub_template` directly from the GitHub repository. See the corresponding [issue on rules_scala][rules_scala_issue] for more information.

This PR replaces `/bin/bash` by `/usr/bin/env bash` in `java_stub_template`.

[rules_scala_issue]: bazelbuild/rules_scala#595

Closes #5979.

PiperOrigin-RevId: 212782646

src/main/java/com/google/devtools/build/lib/bazel/rules/java/java_stub_template.txt

@@ -1,4 +1,4 @@
-#!/bin/bash --posix
+#!/usr/bin/env bash
 # Copyright 2014 The Bazel Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -44,6 +44,8 @@
 #
 # The remainder of the command line is passed to the program.
 
+set -o posix
+
 # Make it easy to insert 'set -x' or similar commands when debugging problems with this script.
 eval "$JAVA_STUB_DEBUG"