[FIX] Resolve pathing issues when running mix via php

[jaxwilko]

While running the MixCompileTest this error was being thrown for all tests:

ERROR in /tests/fixtures/plugins/mix/testa/assets/dist/app
Module not found: Error: Can't resolve '/var/www/vhosts/winter.com/modules/system/assets/src/app.js' in '/var/www/vhosts/winter.com/modules/system'

After debugging, the issue appeared to be:

mix.js('assets/src/app.js', 'assets/dist/app.js');

resolving to: /var/www/vhosts/winter.com/modules/system/assets/src/app.js

Inside the mix object, the publicPath as being correctly set:

"publicPath": "/var/www/vhosts/winter.com/modules/system/tests/fixtures/plugins/mix/testa",

However, when mix created File objects, the absolute path of the file was being passed to path.resolve() which uses process.cwd(), this should be set by the $cwd arg passed to Process however because process was calling npx instead of the webpack bin directly, the cwd was being overwritten. This lead to:

{
    "js": {
      "passive": false,
      "requiresReload": false,
      "caller": "js",
      "toCompile": [
        {
          "entry": [
            {
              "absolutePath": "/var/www/vhosts/winter.com/modules/system/assets/src/app.js",
              "filePath": "assets/src/app.js",
              "segments": {
                "path": "assets/src/app.js",
                "absolutePath": "/var/www/vhosts/winter.com/modules/system/assets/src/app.js",
                "pathWithoutExt": "/var/www/vhosts/winter.com/modules/system/assets/src/app",
                "isDir": false,
                "isFile": true,
                "name": "app",
                "ext": ".js",
                "file": "app.js",
                "base": "/var/www/vhosts/winter.com/modules/system/assets/src"
              }
            }
          ],
          "output": {
            "absolutePath": "/var/www/vhosts/winter.com/modules/system/assets/dist/app.js",
            "filePath": "assets/dist/app.js",
            "segments": {
              "path": "assets/dist/app.js",
              "absolutePath": "/var/www/vhosts/winter.com/modules/system/assets/dist/app.js",
              "pathWithoutExt": "/var/www/vhosts/winter.com/modules/system/assets/dist/app",
              "isDir": false,
              "isFile": true,
              "name": "app",
              "ext": ".js",
              "file": "app.js",
              "base": "/var/www/vhosts/winter.com/modules/system/assets/dist"
            }
          }
        }
      ],
      "activated": true
    },
}

This PR reverts the change using npx back to calling webpack's bin directly.

Linux host test:

Windows host test: