Don't wrap JSON Symbicache hashes in string

This fixes a bug where the hashes were being read in from the file as a string, which throws a runtime error because the Symbicache stores the hashes as a dictionary as opposed to a single string

f4pga/cache.py

@@ -100,4 +100,4 @@ This will cause flow to re-execute from the beggining.""")
     def save(self):
         """Saves cache's state to the persistent storage."""
         with Path(self.cachefile_path).open('w') as wfptr:
-            json_dump(str(self.hashes), wfptr, indent=4)
+            json_dump(self.hashes, wfptr, indent=4)