Custom Command: Add logging for native custom commands

[sonic2kk]

Adds logging for native custom commands, where the output (including any errors) will be stored under STLSHMDIR as launchCustomProg.log. Proton titles already had this loggiing in extProtonRun.

I would also like to find a way to log the PROTON_LOG for Proton custom commands. I'm not sure if it's possible, I think it is from looking at the Proton script - it should default to steam-proton.log if there is no SteamGameId in the environment, and I'm not sure if it will pick up the game AppID by default. If it does, we can temporarily unset this var before launching, and re-set it after launching as a hacky workaround if we really must.

We could even temporarily manipulate the value of SteamGameId to allow us to set the log file to something like steam-<gameid>-customcmd.log so that it is obvious the log is for a custom command pertaining to the given AppID.

Allowing a Proton log has the added benefit of being able to see some Proton-specific environment variables being set. The Proton script has a section where it tries to, for example, log information about the current Steam Linux Runtime. We could re-implement this ourselves, but I'd rather hook into what Proton is currently doing.

All this also hinges on whether or not custom commands running with Proton can even use PROTON_LOG. When using ONLY_CUSTOMCMD in particular I'm not sure how many of our set environment variables (Proton-related or otherwise) get passed to custom commands, for example the ones in the per-game environment variable file, and the global one.

---

This PR does accomplish one logging improvement, but further improvement to be able to give a Proton log since we run custom commands with Proton would be pretty nice.

[sonic2kk]

It seems we can use PROTON_LOG=1, but it won't actually output anything unless we have SteamGameId. This is a problem for two reasons:

• When running from Steam with a game (if we load STL with a game by pressing "Play" and then try to use One-Time Run), we will have SteamGameId but it'll be the same as the game's AppID. So it wouldn't be possible to use both PROTON_LOG for the game and custom command.
• When running outside of Steam (such as by using steamtinkerlaunch otr) then we won't have SteamGameId so the Proton log won't actually get generated.

The Proton Python script does actually have logic that tries to account for generating a Proton log without a SteamGameId, but it only runs if append_forever is True. This seems to be controlled elsewhere, and is not set up by default when running the logic to generate a log.

To get around this, we can probably temporarily set the SteamGameId when running a custom command. The value of the environment variable shouldn't be permanently changed if we set it inline when running the command, but we will need to test this.

So we can actually PROTON_LOG for custom commands. And we could control what the logfile name will be by controlling how we set SteamGameId. But the last problem is cleanly implementing this functionality optionally...

We will probably have to pass an argument to extProtonRun to control if we want Proton logging, off by default. We will then have to add it to each custom command launch case that runs Proton. Finally, we will need to set SteamGameId for the Proton logging.

Adding the ability to PROTON_LOG is probably overkill, I think we are fine for now, but I will add a TODO in case this comes up later or if someone wants to implement it. It would be a nice feature but not one I think is worth delving into right now, as we'd need a lot of custom logic to handle optionally setting SteamGameId and setting it to a custom value (or default if no value is given, such as extProtonRun) if the Proton log is enabled.

I may revisit it in future or someone else might, so a TODO it is. After that, this PR is ready to merge.