common: add LLAMA_LOG_FILE env var (#17609)

Signed-off-by: Aaron Teo <aaron.teo1@ibm.com>

common/arg.cpp

@@ -2639,7 +2639,7 @@ common_params_context common_params_parser_init(common_params & params, llama_ex
         [](common_params &, const std::string & value) {
             common_log_set_file(common_log_main(), value.c_str());
         }
-    ));
+    ).set_env("LLAMA_LOG_FILE"));
     add_opt(common_arg(
         {"--log-colors"}, "[on|off|auto]",
         "Set colored logging ('on', 'off', or 'auto', default: 'auto')\n"