Bug#945263: numpy FTBFS on all

[Rebecca N. Palmer]

Probably because the arch:all build doesn't build python3-numpy (only 
python-numpy-doc), and hence the ls [0] providing the filename for that 
>> evaluates to empty.

The obvious fix (though I haven't tested it) is to wrap this in an "only 
if building arch:any / python3-numpy" conditional.

[0] 
https://salsa.debian.org/python-team/modules/numpy/commit/178302a8042ac98ae4ef5046843579e8b1f4b23a#8756c63497c8dc39f7773438edf53b220c773f67_48_55