Use online checksums for Chisel tool in .NET SDK Dockerfiles

This is part of #6548. Instead of using hard-coded checksums, they should be downloaded from the [Chisel github release](https://github.com/canonical/chisel/releases) where they exist as `.sha384` files. This makes the Dockerfiles more flexible and simplifies this repo's maintenance, since we will no longer need to calculate or keep track of checksums.

Example:
https://github.com/dotnet/dotnet-docker/blob/157267f8403b1985704cb6810a5413fc7e3a1bc2/documentation/ubuntu-chiseled.md?plain=1#L64-L70