Scheduled Linux Library Builds failing: windows-sys conan upload + qt UnicodeDecodeError

## Failures observed

Run: https://github.com/compiler-explorer/infra/actions/runs/24282326593 (2026-04-11)

Two separate jobs fail in the Scheduled Linux Library Builds workflow:

---

### 1. `libraries/rust/windows-sys` — conan upload

The build itself appears to succeed (`windows-sys/0.60.2@windows-sys/0.60.2` package built), but the conan upload to `ceserver` times out or fails. From the logs:

```
Uploading windows-sys/0.60.2@windows-sys/0.60.2 to remote 'ceserver'
Uploading package 1/1: 39864ab616c3262199d8fb953b3cbe5ca3f0c130 to 'ceserver'
```

The job then fails — the upload appears to stall or the ceserver rejects it. This may be related to the previously-known conan proxy/ceserver intermittent issues (see #417).

The original audit summary noted "unknown compiler" — that may be inaccurate; the actual failure is the conan upload step.

---

### 2. `libraries/c++/qt` — UnicodeDecodeError in build logging

When building qt 6.5.2 for `rv64-gcc1520` (riscv64), the build output contains non-UTF-8 bytes that crash the log saving code:

```
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf3 in position 24166: invalid continuation byte
File "/tmp/build/infra/bin/lib/library_builder.py", line 1120, in save_build_logging
```

The fix is to use `errors='replace'` or `errors='ignore'` when reading the build output in `save_build_logging()` in `library_builder.py`.

The underlying build may have also failed (riscv64 cross-compile of Qt is non-trivial), but the crash prevents any useful error from being logged.

---

## Frequency

Both failures observed on 2026-04-11. The windows-sys conan upload issue may be intermittent (ceserver-related). The Qt UnicodeDecodeError is likely consistent for riscv64 targets.

*(I'm Molty, an AI assistant acting on behalf of @mattgodbolt)*