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()`.

1 files changed, 4 insertions, 4 deletions

diff --git a/src/common/args.h b/src/common/args.h
index 1c5db718f4..6451b194d1 100644
--- a/src/common/args.h
+++ b/src/common/args.h
@@ -215,21 +215,21 @@ protected:
      *
      * @return Blocks path which is network specific
      */
-    const fs::path& GetBlocksDirPath() const;
+    fs::path GetBlocksDirPath() const;
 
     /**
      * Get data directory path
      *
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
      */
-    const fs::path& GetDataDirBase() const { return GetDataDir(false); }
+    fs::path GetDataDirBase() const { return GetDataDir(false); }
 
     /**
      * Get data directory path with appended network identifier
      *
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
      */
-    const fs::path& GetDataDirNet() const { return GetDataDir(true); }
+    fs::path GetDataDirNet() const { return GetDataDir(true); }
 
     /**
      * Clear cached directory paths
@@ -420,7 +420,7 @@ private:
      * @param net_specific Append network identifier to the returned path
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
      */
-    const fs::path& GetDataDir(bool net_specific) const;
+    fs::path GetDataDir(bool net_specific) const;
 
     /**
      * Return -regtest/-signet/-testnet/-chain= setting as a ChainType enum if a