.drone.yml: Update npm before install and build

It seems that the issue with not being able to find webpack-cli is
due to some behavior with an older version of npm. My local devel-
opment environment has npm 7.6.x and so does the Node.js 15 conta-
iner, while the Node.js 12 and 14 containers have npm 6.14.x.

Updating npm fixes the issue with `npx webpack` not being able to
find webpack-cli's webpack command (perhaps it installs the peer
dependency automatically, I don't know).

.drone.yml

@@ -6,6 +6,9 @@ steps:
 - name: build
   image: node:12-alpine
   commands:
+  - npm --version
+  - npm install -g npm
+  - npm --version
   - npm install
   - npm run build
 
@@ -18,6 +21,9 @@ steps:
 - name: build
   image: node:14-alpine
   commands:
+  - npm --version
+  - npm install -g npm
+  - npm --version
   - npm install
   - npm run build
 
@@ -30,5 +36,8 @@ steps:
 - name: build
   image: node:15-alpine
   commands:
+  - npm --version
+  - npm install -g npm
+  - npm --version
   - npm install
   - npm run build