Auto merge of #16639 - aneeshusa:skip-nonexisting-dirs-when-cleaning-nightlies, r=jdm

Avoid searching for old nightlies in missing dirs

These directories may be missing (e.g. a first time build or due to Buildbot cleaning),
so don't try to look inside them for old nightlies if so.

I noticed this in http://build.servo.org/builders/arm32/builds/6815/steps/shell__1/logs/stdio.

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python/servo/bootstrap_commands.py

@@ -318,6 +318,8 @@ class MachCommands(CommandBase):
         removing_anything = False
         for tool in ["rust", "cargo"]:
             base = path.join(self.context.sharedir, tool)
+            if not path.isdir(base):
+                continue
             for name in os.listdir(base):
                 # We append `-alt` if LLVM assertions aren't enabled,
                 # so use just the commit hash itself.