Fix the shebang line for bash.

Not all systems having bash in `/bin`.
Use ´/usr/bin/env` to search for bash in `PATH`.

bin/background_jobs

-#!/bin/bash
+#!/usr/bin/env bash
 
 cd $(dirname $0)/..
 

bin/background_jobs_sk

-#!/bin/bash
+#!/usr/bin/env bash
 
 cd $(dirname $0)/..
 app_root=$(pwd)

bin/background_jobs_sk_cluster

-#!/bin/bash
+#!/usr/bin/env bash
 
 cd $(dirname $0)/..
 app_root=$(pwd)

scripts/clean-old-cached-assets

-#!/bin/bash
+#!/usr/bin/env bash
 
 # Clean up cached files that are older than 4 days
 find tmp/cache/assets/sprockets/ -type f -mtime +4 -execdir rm -- "{}" \;

scripts/create_postgres_user.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 psql -h postgres -U postgres postgres <<EOF
 CREATE USER gitlab;

scripts/prepare_postgres_fdw.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 psql -h postgres -U postgres gitlabhq_geo_test <<EOF
 CREATE EXTENSION postgres_fdw;

scripts/review_apps/gcp_cleanup.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 source scripts/utils.sh
 

scripts/rspec_helpers.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 function retrieve_tests_metadata() {
   mkdir -p knapsack/ rspec_flaky/ rspec_profiling/

scripts/security-harness

@@ -19,7 +19,7 @@ end
 
 HOOK_PATH = File.expand_path("../.git/hooks/pre-push", __dir__)
 HOOK_DATA = <<~HOOK
-  #!/bin/bash
+  #!/usr/bin/env bash
 
   set -e
 

vendor/elastic_stack/wait-for-elasticsearch.sh

-#!/bin/bash
+#!/usr/bin/env bash
 # http://redsymbol.net/articles/unofficial-bash-strict-mode/
 IFS=$'\n\t'
 set -euo pipefail