tests/utils/env.sh.in: remove `#!/bin/bash`, add a ShellCheck directive

tests/utils/env.sh.in: remove `#!/bin/bash`, add a ShellCheck directive

`tests/utils/env.sh` is only meant to be sourced, never executed
directly, therefore the `#!/bin/bash` line is useless.

Add a ShellCheck shell directive to tell ShellCheck that this is (at
least) Bash code.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ieb9bd996e43e8609d1e3e7b7b3d614bd4db6f234