fix(ci): adjust test-core's downgrade step (#7796)

.github/workflows/test-core.yml

@@ -102,7 +102,7 @@ jobs:
         # The --precise flag can only be used once per invocation.
         run: |
           cargo update -p time --precise 0.3.23
-          cargo update -p toml@0.7.7 --precise 0.7.6
+          cargo update -p toml@0.7.8 --precise 0.7.6
           cargo update -p toml_edit@0.19.15 --precise 0.19.14
           cargo update -p cfg-expr@0.15.5 --precise 0.15.4