Reset PyInstaller environment on :restart

Starting with PyInstaller 6.10 (6.9?), we're supposed to tell PyInstaller when
we restart our application (and a subprocess should outlive this process).

In their words:

    The above requirement was introduced in PyInstaller 6.9, which changed the
    way the bootloader treats a process spawned via the same executable as its
    parent process. Whereas previously the default assumption was that it is
    running a new instance of (the same) program, the new assumption is that the
    spawned process is some sort of a worker subprocess that can reuse the
    already-unpacked resources. This change was done because the worker-process
    scenarios are more common, and more difficult to explicitly accommodate
    across various multiprocessing frameworks and other code that spawns worker
    processes via sys.executable.

https://pyinstaller.org/en/stable/common-issues-and-pitfalls.html#independent-subprocess
https://pyinstaller.org/en/stable/CHANGES.html (6.10)

While from a quick test on Windows, things still worked without setting the
variable (possibly because we don't use a onefile build), it still seems
reasonable to do what PyInstaller recommends doing.

Follow-up to #8269.

qutebrowser/misc/quitter.py

@@ -177,10 +177,17 @@ class Quitter(QObject):
         assert ipc.server is not None
         ipc.server.shutdown()
 
+        if hasattr(sys, 'frozen'):
+            # https://pyinstaller.org/en/stable/common-issues-and-pitfalls.html#independent-subprocess
+            env = os.environ.copy()
+            env["PYINSTALLER_RESET_ENVIRONMENT"] = "1"
+        else:
+            env = None
+
         # Open a new process and immediately shutdown the existing one
         try:
             args = self._get_restart_args(pages, session, override_args)
-            proc = subprocess.Popen(args)  # pylint: disable=consider-using-with
+            proc = subprocess.Popen(args, env=env)  # pylint: disable=consider-using-with
         except OSError:
             log.destroy.exception("Failed to restart")
             return False