Run node-gyp and node-gyp-build via npx

Otherwise system versions are run and trigger
nodejs/node-gyp#2005 bug which is already
fixed in node-gyp 6.1.0.

package.json

@@ -7,10 +7,10 @@
     "coverage-html-report": "rm -rf coverage/ && nyc report --reporter=html && opn coverage/index.html",
     "generate-constants": "./scripts/generate-constants.js",
     "install": "node scripts/install.js",
-    "node-gyp-build": "node-gyp-build \"node scripts/rebuild-core.js\" \"node scripts/postinstall.js\"",
+    "node-gyp-build": "npx node-gyp-build \"node scripts/rebuild-core.js\" \"node scripts/postinstall.js\"",
     "rebuild-all": "npm run rebuild-core && npm run rebuild-bindings",
     "rebuild-core": "node scripts/rebuild-core.js",
-    "rebuild-bindings": "node-gyp rebuild && npm run generate-constants",
+    "rebuild-bindings": "npx node-gyp rebuild && npm run generate-constants",
     "prebuild": "prebuildify -t 10.16.0 --napi --strip --postinstall \"node scripts/postinstall.js --prebuild\"",
     "download-prebuilds": "prebuildify-ci download",
     "submodule": "git submodule update --recursive --init",