build: use c8 for coverage

package.json

@@ -28,8 +28,8 @@
     "prepare": "npm run compile",
     "samples-test": "cd samples/ && npm link ../ && npm install && npm test && cd ../",
     "presystem-test": "npm run compile",
-    "system-test": "nyc mocha build/system-test --timeout 600000",
-    "test": "nyc cross-env CLOUD_DEBUG_ASSERTIONS=1 mocha build/test",
+    "system-test": "c8 mocha build/system-test --timeout 600000",
+    "test": "c8 cross-env CLOUD_DEBUG_ASSERTIONS=1 mocha build/test",
     "compile": "tsc -p .",
     "postcompile": "cpy test/*.js test/**/*.js test/fixtures build --parents",
     "fix": "gts fix",
@@ -80,8 +80,8 @@
     "mocha": "^8.0.0",
     "mv": "^2.1.1",
     "ncp": "^2.0.0",
+    "c8": "^7.2.1",
     "nock": "^13.0.0",
-    "nyc": "^14.0.0",
     "proxyquire": "^2.0.0",
     "teeny-request": "^7.0.0",
     "tmp-promise": "^3.0.0",