fix(check_updates): do not fail with a strange error message

Reading the date of the last check from update_check.yaml failed because
the name of the key is `check-at`, not `check_at`. This used to work
because `load_yaml_dict()` transparently turned `-` into `_`, but it
does not do this anymore now that we fixed #480.

ggshield/core/check_updates.py

@@ -17,6 +17,8 @@
     "update_check.yaml",
 )
 
+CHECK_AT_KEY = "check-at"
+
 
 # Use a short timeout to prevent blocking
 CHECK_TIMEOUT = 5
@@ -26,6 +28,11 @@ def _split_version(version: str) -> Tuple[int, ...]:
     return tuple([int(x) for x in version.split(".")])
 
 
+def _save_check_time(check_at: float) -> None:
+    """Store `check_at` in our cache"""
+    save_yaml_dict({CHECK_AT_KEY: check_at}, CACHE_FILE)
+
+
 def check_for_updates() -> Optional[str]:
     """
     Check for ggshield updates on GitHub. Return the latest version if available.
@@ -40,7 +47,7 @@ def check_for_updates() -> Optional[str]:
         cached_data = None
     if cached_data is not None:
         try:
-            check_at = cached_data["check_at"]
+            check_at = cached_data[CHECK_AT_KEY]
         except Exception as e:
             logger.warning("Could not load cached latest version: %s", repr(e))
 
@@ -53,7 +60,7 @@ def check_for_updates() -> Optional[str]:
     # Save check time now so that it is saved even if the check fails. This ensures we
     # don't try for every command if the user does not have network access.
     try:
-        save_yaml_dict({"check_at": time.time()}, CACHE_FILE)
+        _save_check_time(time.time())
     except Exception as e:
         logger.warning("Could not save time of version check to cache: %s", repr(e))
         # Do not continue if we can't save check time. If we continue we are going to
@@ -89,7 +96,7 @@ def check_for_updates() -> Optional[str]:
                 return None
 
             try:
-                save_yaml_dict({"check_at": check_at}, CACHE_FILE)
+                _save_check_time(check_at)
             except Exception as e:
                 logger.warning(
                     "Could not save time of version check to cache: %s", repr(e)