[firtool] Firtool 1.138.0 failed to create diretories on Windows

Platform: Windows 11, x64
Firtool: 1.138.0
[test.txt](https://github.com/user-attachments/files/24074662/test.txt)
Comman line:
```
firtool --split-verilog --format fir test.txt -o rtl
```

Logs:
```
firtool-1.138.0 : test.txt:0:0: error: cannot create output directory "rtl\verificationsert": No mapping for the Unicode character exists in 
the target multi-byte code page.

...
```