🐛 Fix build by setting node v16

The error was:
```
Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './lib/tokenize' is not defined by "exports" in /home/runner/work/gitpop3/gitpop3/node_modules/postcss-safe-parser/node_modules/postcss/package.json
    at new NodeError (node:internal/errors:405:5)
    at exportsNotFound (node:internal/modules/esm/resolve:366:10)
    at packageExportsResolve (node:internal/modules/esm/resolve:713:9)
    at resolveExports (node:internal/modules/cjs/loader:584:36)
    at Module._findPath (node:internal/modules/cjs/loader:658:31)
    at Module._resolveFilename (node:internal/modules/cjs/loader:1120:27)
    at Module._load (node:internal/modules/cjs/loader:975:27)
    at Module.require (node:internal/modules/cjs/loader:1225:19)
    at require (node:internal/modules/helpers:177:18)
    at Object.<anonymous> (/home/runner/work/gitpop3/gitpop3/node_modules/postcss-safe-parser/lib/safe-parser.js:1:17) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}
```

.github/workflows/deploy.yml

@@ -11,6 +11,8 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-node@v2
+      with:
+        node-version: '16.x'
     - name: git config
       run: |
         git config user.name github-pages-deploy-action