proton: Make log creation depend only on PROTON_LOG.

There are valid cases, e.g. when debugging Wine Mono, to have WINEDEBUG
set to -all. Before this change this would disable the logging even if
PROTON_LOG=1.

proton

@@ -1391,7 +1391,7 @@ class Session:
         if "PROTON_CRASH_REPORT_DIR" in self.env:
             self.env["WINE_CRASH_REPORT_DIR"] = self.env["PROTON_CRASH_REPORT_DIR"]
 
-        if self.env["WINEDEBUG"] != "-all":
+        if "PROTON_LOG" in self.env and nonzero(self.env["PROTON_LOG"]):
             if self.setup_logging(append_forever=False):
                 self.log_file.write("======================\n")
                 with open(g_proton.version_file, "r") as f:

user_settings.sample.py

@@ -6,6 +6,9 @@ user_settings = {
     #By default, logs are saved to $HOME/steam-<STEAM_GAME_ID>.log, overwriting any previous log with that name.
     #Log directory can be overridden with $PROTON_LOG_DIR.
 
+    #enable logging
+    "PROTON_LOG": "1",
+
     #Wine debug logging
     "WINEDEBUG": "+timestamp,+pid,+seh,+unwind,+debugstr,+loaddll,+mscoree",