Convert some C-style strings in uemis-downloader.cpp to std::string.
This has the side effect of fixing builds on Debian Trixie, which
currently fail with the (rather silly) error:
/build/subsurface-beta-202405060411/core/uemis-downloader.cpp: In function 'char* build_ans_path(const char*, int)':
/build/subsurface-beta-202405060411/core/uemis-downloader.cpp:290:32: error: '%s' directive output between 0 and 12 bytes may cause result to exceed 'INT_MAX' [-Werror=format-truncation=]
290 | snprintf(buf, len, "%s/%s", path, name);
| ^~
......
529 | ans_path = build_filename(intermediate, fl);
| ~~
cc1plus: some warnings being treated as errors
Signed-off-by: Richard Fuchs <dfx@dfx.at>