"yarn run build" fails on Windows
### Summary

Running the `yarn run build` script fails on Windows.

### Steps to reproduce

1) Download the postgraphile repo, on Windows.
2) Open a terminal in the repo root.
3) Run `yarn`, to install the dependencies. (not actually necessary to demonstrate this issue, but might as well)
4) Run: `yarn run build`

### Expected results

I expect to see a success message in the terminal, and the outputted files in the `build-turbo` folder.

### Actual results

```
PS C:\Root\Apps\@V\@Modules\postgraphile\Fork> yarn run build
yarn run v1.22.10
$ ./scripts/build
'.\scripts\build' is not recognized as an internal or external command,
operable program or batch file.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
```

Same thing for NPM:
```
PS C:\Root\Apps\@V\@Modules\postgraphile\Fork> npm run build

> postgraphile@4.12.0-alpha.0 build C:\Root\Apps\@V\@Modules\postgraphile\Fork
> ./scripts/build

'.' is not recognized as an internal or external command,
operable program or batch file.
npm ERR! code ELIFECYCLE [...]
```

I also tried renaming the scripts to end with `.bat` and running them in cmd/powershell, but that (as expected) fails as well. (many errors, due to linux commands like `rm` not being present)

### Additional context

OS: Windows 10
Node: v14.5.0

### Possible Solution

Switch from using bash scripts to NodeJS scripts, or some other cross-platform scripting solution (eg. based on [shx](https://github.com/shelljs/shx) or [shelljs](https://github.com/shelljs/shelljs)).