revert use of npm exec

package.json

@@ -20,8 +20,8 @@
     "test:te": "TEXT_ENCODING=force mocha 'test/**/*.test.ts'",
     "test:dist:purejs": "TS_NODE_PROJECT=tsconfig.test-dist-es5-purejs.json npm run test:purejs -- --reporter=dot",
     "test:cover": "npm run cover:clean && npm-run-all 'test:cover:*' && npm run cover:report",
-    "test:cover:purejs": "npm exec nyc --no-clean npm run test:purejs",
-    "test:cover:te": "npm exec nyc --no-clean npm run test:te",
+    "test:cover:purejs": "npx nyc --no-clean npm run test:purejs",
+    "test:cover:te": "npx nyc --no-clean npm run test:te",
     "test:deno": "deno test test/deno_test.ts",
     "test:fuzz": "npm exec -- jsfuzz@git+https://gitlab.com/gitlab-org/security-products/analyzers/fuzzers/jsfuzz.git --fuzzTime 60 --no-versifier test/decode.jsfuzz.js corpus",
     "cover:clean": "rimraf .nyc_output coverage/",