fix missing printf specifiers

fixes these warnings (from !54 (merged)):

../src/main.c: In function 'capture_completed':
../src/main.c:321:9: error: format not a string literal and no format arguments -Werror=format-security
321 | snprintf(last_path, sizeof(last_path), args->fname);

../src/process_pipeline.c: In function 'setup_capture':
../src/process_pipeline.c:165:9: error: format not a string literal and no format arguments [-Werror=format-security]
165 | snprintf(burst_dir, sizeof(burst_dir), tempdir);

otherwise, args->fname and tempdir themself would be interpreted as the specifier.