package.json: Run webpack with npx

My local dev environment automatically adds ./node_modules/.bin to
the shell PATH, but it seems a better way to do this is to use npx
so that it works here and on the CI.

.drone.yml

@@ -1,17 +1,5 @@
 kind: pipeline
 type: docker
-name: node10
-
-steps:
-- name: build
-  image: node:10-alpine
-  commands:
-  - npm install
-  - npm run build
-
----
-kind: pipeline
-type: docker
 name: node12
 
 steps:
@@ -32,3 +20,15 @@ steps:
   commands:
   - npm install
   - npm run build
+
+---
+kind: pipeline
+type: docker
+name: node15
+
+steps:
+- name: build
+  image: node:15-alpine
+  commands:
+  - npm install
+  - npm run build

package.json

@@ -10,7 +10,7 @@
     "build:css": "sass --style expanded source/scss/style.scss assets/css/style.css.tmp && cleancss --level 1 assets/css/style.css.tmp -o assets/css/style.css",
     "build:rtlcss": "rtlcss -s assets/css/style.css.tmp assets/css/style.rtl.css.tmp && cleancss --level 1 assets/css/style.rtl.css.tmp -o assets/css/style.rtl.css",
     "build:cookieconsent": "cp node_modules/@chiiya/haven/dist/haven.umd.min.js assets/js",
-    "build:js": "webpack",
+    "build:js": "npx webpack",
     "build": "npm run build:css && npm run build:rtlcss && npm run build:js && npm run build:cookieconsent && npm run clean",
     "clean": "rm assets/css/style.css.tmp assets/css/style.css.tmp.map assets/css/style.rtl.css.tmp"
   },