nixos/lib/test-driver: try using XDG_RUNTIME_DIR if available

At work we have the use-case that several people connect to a large
Linux box to run tests and debug those interactively.

All tests write their state into a global `/tmp` -- e.g. the vde1 socket
and the VMs' state. This leads to conflicts when multiple people are
doing this.

This change tries to use XDG_RUNTIME_DIR before using Python's detection
of a global temp directory: when connecting, this requires a working
user session, but then we get working directories per user. This is
preferable over doing something like `mktemp -d` per run since that
would break use-cases where you want to keep the VMs' state across
multiple sessions (`--keep-vm-state`).

(cherry picked from commit 59b4d0de90)

nixos/lib/test-driver/src/test_driver/driver.py

@@ -39,15 +39,15 @@ def get_tmp_dir() -> Path:
     Raises an exception in case the retrieved temporary directory is not writeable
     See https://docs.python.org/3/library/tempfile.html#tempfile.gettempdir
     """
-    tmp_dir = Path(tempfile.gettempdir())
+    tmp_dir = Path(os.environ.get("XDG_RUNTIME_DIR", tempfile.gettempdir()))
     tmp_dir.mkdir(mode=0o700, exist_ok=True)
     if not tmp_dir.is_dir():
         raise NotADirectoryError(
-            f"The directory defined by TMPDIR, TEMP, TMP or CWD: {tmp_dir} is not a directory"
+            f"The directory defined by XDG_RUNTIME_DIR, TMPDIR, TEMP, TMP or CWD: {tmp_dir} is not a directory"
         )
     if not os.access(tmp_dir, os.W_OK):
         raise PermissionError(
-            f"The directory defined by TMPDIR, TEMP, TMP, or CWD: {tmp_dir} is not writeable"
+            f"The directory defined by XDG_RUNTIME_DIR, TMPDIR, TEMP, TMP, or CWD: {tmp_dir} is not writeable"
         )
     return tmp_dir