chore: change shebang in link_all.sh to be portable (aws#33520)

### Reason for this change

Current shebang in `link_all.sh` is `!#/bin/bash`, which does not work if the `bash` binary is not present in `/bin` (such is the case on e.g. NixOS).

### Description of changes

Change the sheband to `!#/usr/bin/env bash`, so that bash is executed from wherever it's present, not necessarily `/bin/bash`.

### Description of how you validated changes

The current script fails on NixOS for me.
The script with the updated shebang suceeds.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

link-all.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # creates symlinks under node_modules for all packages in this repo
 # can be used to work against this code base.
 set -euo pipefail