Make install.sh shebang more generic (#46)

#### Problem

The install script doesn't work on setups which put `bash` in a different place than `/bin`. For example, nixos doesn't put its bins in there.

#### Summary of changes

Use the more generic shebang of `#!/usr/bin/env bash` to allow the system to find it instead.

install.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -euo pipefail
 
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