ArgsManager: return path by value from GetBlocksDirPath()

`ArgsManager::m_cached_blocks_path` is protected by
`ArgsManager::cs_args` and returning a reference to it after releasing
the mutex is unsafe.

To resolve this, return a copy of the path. This has some performance
penalty which is presumably ok, given that paths are a few 100s bytes
at most and `GetBlocksDirPath()` is not called often.

This silences the following (clang 18):

```
common/args.cpp:288:31: error: returning variable 'm_cached_blocks_path' by reference requires holding mutex 'cs_args' [-Werror,-Wthread-safety-reference-return]
  288 |     if (!path.empty()) return path;
      |                               ^
```

Do the same with
`ArgsManager::GetDataDir()`,
`ArgsManager::GetDataDirBase()` and
`ArgsManager::GetDataDirNet()`.

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