[tools] Let gcc work in the sandbox.

BUGS

@@ -7,7 +7,6 @@ Current
 - If a job is paused for longer than the timeout period, sometimes it has
   the timed out status when un-paused.
 - Tmp files are being left behind after shutdown.
-- gcc is not working inside the sandbox.
 
 
 Current (tool related)
@@ -243,6 +242,8 @@ Fixed
 - When shutting down "Exception ignored in:" noise appears. Four messages
   for every worker running. This started with the move to asyncio for the
   workers.
+- gcc is not working inside the sandbox.
+  <- Let /dev/null through the sandbox.
 
 
 Won't fix

sandbox_fs.py

@@ -59,6 +59,7 @@ class SandboxFs:
 
     def __init__(self, holes=None):
         self.holes = [] if holes is None else holes
+        self.holes += ["/dev/null"]
         for hole in self.holes:
             if not hole.startswith("/"):
                 raise ValueError("Holes must be absolute paths: %r" % hole)