fail to create diagnostic file when -b is used
turris-diagnostics fails to create file when `-b` is used.
Foris-controller uses `-b` to suppress output, which won't be read anyway.
## How to reproduce
* turris-diagnostics: 13.1
* TOS: HBL-5.2.0
```
/usr/share/diagnostics/diagnostics.sh -b -o /tmp/diagnostics-deadbeef.out firewall
```
## Expected result
```
root@turris:~# ls /tmp/diagnostics*
diagnostics-deadbeef.out
```
## Actual result
```
root@turris:~# ls /tmp/diagnostics-*
ls: /tmp/diagnostics-*: No such file or directory
```
## What works
```
/usr/share/diagnostics/diagnostics.sh -o /tmp/diagnostics-deadbeef.out firewall
```
issue