nixos/nvidia: Set SidebandSocketPath to a user-writable path in `/run`

The NVIDIA X driver uses a UNIX domain socket to pass information to
other driver components. If unable to connect to this socket, some
driver features, such as G-Sync, may not work correctly. The socket will
be bound to a file with a name unique to the X server instance created
in the directory specified by this option. Note that on Linux, an
additional abstract socket (not associated with a file) will also be
created, with this pathname socket serving as a fallback if connecting
to the abstract socket fails.

The default, which was in effect prior to this change, was `/var/run`.

The effect of not setting this option was that GDM X sessions
(and other non-root sessions) would see this warning in the log files:

```
   (WW) NVIDIA: Failed to bind sideband socket to
   (WW) NVIDIA:     '/var/run/nvidia-xdriver-b4f69129' Permission denied
```

I don't see any security implications of turning this on universally,
since there already was an abstract socket created according to the
docs.

Documentation:

1. [NVIDIA X Config Options](https://download.nvidia.com/XFree86/Linux-x86_64/440.82/README/xconfigoptions.html#SidebandSocketPath)

Diagnosis:

1. [Arch Linux BBS post](https://bbs.archlinux.org/viewtopic.php?pid=1909115#p1909115)

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