fix: fix missing await when opening checkpoint output (#2868)

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### QC
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* [x] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [x] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).

snakemake/io.py

@@ -327,7 +327,7 @@ def open(self, mode="r", buffering=-1, encoding=None, errors=None, newline=None)
         This can (and should) be used in a `with`-statement.
         If the file is a remote storage file, retrieve it first if necessary.
         """
-        if self.is_storage and not self.exists_local():
+        if self.is_storage and not async_run(self.exists_local()):
             async_run(self.retrieve_from_storage())
         f = open(
             self,