minorbug(shred): using --random-source /var/run/snapd.socket, it returns a generic `cannot open` error

Hi, uutils mainteners

i see when passing --random-source to that argument **/var/run/snapd.socket**, as our testing confirms
the uu shred does just return vague cannot open error message

```css
relunsec@relunsec:~/software/coreutils/target/debug$ ./shred --random-source /var/run/snapd.socket a
shred: cannot open random source: '/var/run/snapd.socket'
```