Check all shebangs for consistency

[pdehaan]

In the spirit of #2393 (yay adding file extensions to scripts when possible), it'd probably be a good idea to quickly verify all the shebangs are correct in the ./bin/* files. I noticed in some places we do #!/bin/bash and in others we do #!/usr/bin/env bash:

$ git grep "#\!" bin

bin/_run-docker:#!/bin/bash
bin/build-docker-image:#!/usr/bin/env bash
bin/build-scripts/bundle_dependencies:#!/usr/bin/env bash
bin/build-scripts/css_to_js:#!/usr/bin/env python
bin/build-scripts/fixup_panel_js:#!/usr/bin/env python
bin/build-scripts/modularize:#!/usr/bin/env bash
bin/build-scripts/pontoon-to-webext.js:#! /usr/bin/env node
bin/build-scripts/set_file:#!/usr/bin/env bash
bin/build-scripts/sub_rdf_checkout_version:#!/usr/bin/env python
bin/build-scripts/update_manifest:#!/usr/bin/env python
bin/build-scripts/write_build_time:#!/usr/bin/env python
bin/build-scripts/write_ga_id:#!/usr/bin/env node
bin/clean-checkout:#!/usr/bin/env bash
bin/commit-new-version:#!/usr/bin/env bash
bin/dumpschema:#!/usr/bin/env bash
bin/generate-commit-log:#!/usr/bin/env python
bin/load_test_exercise.py:#!/usr/bin/env python
bin/make-data-url:#!/usr/bin/env python
bin/pg_vars:#!/usr/bin/env node
bin/release-version:#!/usr/bin/env bash
bin/run-addon:#!/bin/bash
bin/run-server:#!/bin/bash
bin/test-request:#!/usr/bin/env bash