misc: Use /usr/bin/env for a more generic shebang

Use path based SHELL in Makefiles. It was suggested that this is a
better solution for make for cases when there is no /usr/bin/env.

See: https://github.com/purpleidea/mgmt/pull/694#discussion_r1015596204

test/test-shell.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # simple test harness for testing mgmt
 # NOTE: this will rm -rf /tmp/mgmt/