Replace all usages of /bin/bash shebang with /usr/bin/env (#40756)

Replace all usages of /bin/bash with /usr/bin/env

/usr/bin/env is nearly guaranteed to always exist at that location, which can't be said about /bin/bash and /bin/sh.

Co-authored-by: opl <4833621+opl@users.noreply.github.com>

BuildChecker/hooks/post-checkout

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 gitroot=$(git rev-parse --show-toplevel)
 

BuildChecker/hooks/post-merge

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Just call post-checkout since it does the same thing.
 gitroot=$(git rev-parse --git-path hooks)