gradlew: 'sh' is not recognized as an internal or external command,

# Bug Report

## Problem

When I run the following command in PowerShell on Windows 10, I get an error:

```
cordova build android --verbose

=>
...
Running command: C:\MyPath\platforms\android\gradlew cdvBuildDebug -b C:\MyPath\platforms\android\build.gradle
'sh' is not recognized as an internal or external command,
operable program or batch file.
Command finished with error code 1: C:\MyPath\platforms\android\gradlew cdvBuildDebug,-b,C:\MyPath\platforms\android\build.gradle
C:\MyPath\platforms\android\gradlew: Command failed with exit code 1 Error output:
'sh' is not recognized as an internal or external command,
operable program or batch file.
Error: C:\MyPath\platforms\android\gradlew: Command failed with exit code 1 Error output:
'sh' is not recognized as an internal or external command,
operable program or batch file.
at ChildProcess.whenDone (C:\MyPath\node_modules\cordova-common\src\superspawn.js:135:23)
at ChildProcess.emit (events.js:315:20)
at ChildProcess.cp.emit (C:\MyPath\node_modules\cross-spawn\lib\enoent.js:34:29)
at maybeClose (internal/child_process.js:1021:16)
at Process.ChildProcess._handle.onexit (internal/child_process.js:286:5)
```

### What is expected to happen?

I am able to run this command on Ubuntu (non-WSL) and it successfully finds the correct version of `gradlew`. However, in Windows, I think it should find `gradlew.bat`.

### What does actually happen?

It runs `gradlew` on Windows, rather than `gradlew.bat`

## Information

`check_reqs.js` locates Android Studio and then gradle in this location:

```
C:\Program Files\Android\Android Studio\gradle\gradle-4.6\bin\gradle
```

However, `ProjectBuilder.js` [shows this](https://github.com/apache/cordova-android/blob/master/bin/templates/cordova/lib/builders/ProjectBuilder.js#L347):

```javascript
build (opts) {
var wrapper = path.join(this.root, 'gradlew');
...
```

It looks to me like there is no attempt to append `.bat` to the filename if it's on Windows. Maybe [execa](https://github.com/sindresorhus/execa) is supposed to append it, but it doesn't seem to. If I hack this to 'gradlew.bat', it builds successfully.

### Command or Code

```
cordova build android --verbose
```

### Environment, Platform, Device

Windows 10

### Version information

Cordova 9.0.0
It seems to find Android Studio 3.2.1, although I'm not actually using it.

## Checklist

- [X] I searched for existing GitHub issues
- [X] I updated all Cordova tooling to most recent version
- [X] I included all the necessary information above

Start in bin/templates/cordova/lib/builders/ProjectBuilder.js around the build method shown in the issue, then reproduce with `cordova build android --verbose` on Windows 10. Check how the Android Gradle wrapper is selected and verify that the Windows build invokes gradlew.bat and completes successfully.

Written by the indexing model from the issue text.