Use release environment when building with swift

[nathanfallet]

https://github.com/netlify/build-image/blob/30f629161c0736b1a3ecd8b418e5eeffab5c0faf/run-build-functions.sh#L440

I thing this command build for debug by default, this should be done by using swift build -c release instead. This has a huge performance impact (using release can speed up a lot). Same thing if swift run is used, should be replaced by swift run -c release too.