support sparse checkout: ./swift/utils/update-checkout

**Motivation**
swift source code is big and takes long especially for an initial checkout.

**Solution**
nowadays sparse checkout is a good option to speed up with (nearly) no disadvantages in daily work:
git clone --filter=tree:0 ...

maybe like:
```
GITFLAGS="--filter=tree:0" ./swift/utils/update-checkout
```


**Alternatives considered**
none known.