cacheprovider: set: use json.dumps + write

``json.dump`` is slower since it iterates over chunks [1].

For 100 ``cache.set`` calls this saved ~0.5s (2.5s => 2s), using a dict
with 1500 entries, and an encoded size of 500kb.

Python 3.7.4.

1: https://github.com/blueyed/cpython/blob/1c2e81ed00/Lib/json/__init__.py#L177-L180

changelog/6206.improvement.rst

@@ -0,0 +1 @@
+cacheprovider: improved robustness and performance with ``cache.set``.

src/_pytest/cacheprovider.py

@@ -125,13 +125,14 @@ class Cache:
             return
         if not cache_dir_exists_already:
             self._ensure_supporting_files()
+        data = json.dumps(value, indent=2, sort_keys=True)
         try:
             f = path.open("w")
         except (IOError, OSError):
             self.warn("cache could not write path {path}", path=path)
         else:
             with f:
-                json.dump(value, f, indent=2, sort_keys=True)
+                f.write(data)
 
     def _ensure_supporting_files(self):
         """Create supporting files in the cache dir that are not really part of the cache."""