make vm-build-freebsd appears to require . in PATH

[Markus Armbruster]

Look at this:

    $ make vm-build-freebsd
    [...]
    ### Preparing iso and disk image ...
    /home/armbru/.cache/qemu-vm/images/freebsd.img.install.iso.xz (1/1)
      100 %       684.3 MiB / 959.4 MiB = 0.713   222 MiB/s       0:04          
   
    Failed to prepare guest environment
    Traceback (most recent call last):
      File "/work/armbru/qemu/tests/vm/basevm.py", line 640, in main
        return vm.build_image(args.image)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/work/armbru/qemu/tests/vm/freebsd", line 100, in build_image
        self.exec_qemu_img("create", "-f", "qcow2", img_tmp, self.size)
      File "/work/armbru/qemu/tests/vm/basevm.py", line 270, in exec_qemu_img
        subprocess.check_call(cmd)
      File "/usr/lib64/python3.11/subprocess.py", line 408, in check_call
        retcode = call(*popenargs, **kwargs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.11/subprocess.py", line 389, in call
        with Popen(*popenargs, **kwargs) as p:
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.11/subprocess.py", line 1024, in __init__
        self._execute_child(args, executable, preexec_fn, close_fds,
      File "/usr/lib64/python3.11/subprocess.py", line 1901, in _execute_child
        raise child_exception_type(errno_num, err_msg, err_filename)
    FileNotFoundError: [Errno 2] No such file or directory: 'qemu-img'
    make: *** [/work/armbru/qemu/tests/vm/Makefile.include:97: 
/home/armbru/.cache/qemu-vm/images/freebsd.img] Error 2
    make: Target 'vm-build-freebsd' not remade because of errors.
    make: Leaving directory '/work/armbru/qemu/bld'

Or similar errors at a later stage when the "guest environment" doesn't
need preparing anymore (I guess).

If I add PATH="$PATH:.", I don't get such errors.