Introduce GetUniquePath(base) helper method to replace boost::filesystem::unique_path() which is not available in std::filesystem.

src/util/system.cpp

@@ -12,6 +12,7 @@
 #include <chainparamsbase.h>
 #include <sync.h>
 #include <util/check.h>
+#include <util/getuniquepath.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 #include <util/translation.h>
@@ -124,7 +125,7 @@ void ReleaseDirectoryLocks()
 
 bool DirIsWritable(const fs::path& directory)
 {
-    fs::path tmpFile = directory / fs::unique_path();
+    fs::path tmpFile = GetUniquePath(directory);
 
     FILE* file = fsbridge::fopen(tmpFile, "a");
     if (!file) return false;