fuzz: psbt_base64_decode fails on Windows

https://github.com/bitcoin/bitcoin/actions/runs/14046520411/job/39356515207

```

Assertion failed: DecodeBase64PSBT(psbt, random_string, error) == error.empty(), file D:\a\bitcoin\bitcoin\src\test\fuzz\base_encode_decode.cpp, line 95
Error processing input "D:\\a\\_temp\\qa-assets\\fuzz_corpora\\psbt_base64_decode\\0041ae3fba09a33e18a330c95acce904552b7114"