FIXUP: store in base isolation_provider the stderr

Implemented per suggestion [1].
[1]: https://github.com/freedomofpress/dangerzone/pull/627/files#r1413959821

dangerzone/isolation_provider/base.py

@@ -55,6 +55,11 @@ def __init__(self) -> None:
         self.percentage = 0.0
         self.proc: Optional[subprocess.Popen] = None
 
+        if getattr(sys, "dangerzone_dev", False) == True:
+            self.proc_stderr = subprocess.PIPE
+        else:
+            self.proc_stderr = subprocess.DEVNULL
+
     @abstractmethod
     def install(self) -> bool:
         pass

dangerzone/isolation_provider/container.py

@@ -166,17 +166,11 @@ def exec(
         args_str = " ".join(shlex.quote(s) for s in args)
         log.info("> " + args_str)
 
-        dev_mode = getattr(sys, "dangerzone_dev", False) == True
-        if dev_mode:
-            stderr = subprocess.PIPE
-        else:
-            stderr = subprocess.DEVNULL
-
         return subprocess.Popen(
             args,
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
-            stderr=stderr,
+            stderr=self.proc_stderr,
             startupinfo=startupinfo,
         )