Fixed usage of fmt::format and fmt::print for C++20 (#323)

With C++20 `fmt::print(s)` expects a compile time format string and
otherwise fails complaining about consteval. To get runtime formatting
use `fmt::print(fmt::runtime(s))`

src/RawSubFile.cpp

@@ -167,7 +167,7 @@ void RawSubFile::parse_fname(const std::filesystem::path &fname) {
 }
 
 std::filesystem::path RawSubFile::fpath(size_t file_index) const {
-    auto fname = fmt::format(m_base_name, file_index);
+    auto fname = fmt::format(fmt::runtime(m_base_name), file_index);
     return m_path / fname;
 }