Change executables to use `#!/usr/bin/env bash`

This is more or less recommended as a portability standard. All BSD and
Linux distros use this, as do quite a few other *nix systems.

bin/linkup

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -euo pipefail
 IFS=$'\n\t'
 

bin/show-versions

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -euo pipefail
 IFS=$'\n\t'
 

bin/version-compare

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -euo pipefail
 
 # Compare 2 version strings.

bin/zfs-backup

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -euo pipefail
 IFS=$'\n\t'
 

vim/templates/bashscript

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # The Unofficial "Bash Strict Mode".
 #