[buildpacks] build dotnet-amd64 on arm64 machine

Bug Report

Currently, pack doesn't support specific platform targeting. However, it should still be possible to produce an amd64 image on an arm64 machine.

Current behavior

On M1 Mac, with dotnet and pack installed:

1. dotnet new web -o repro
2. cd repro
3. pack build --builder "mcr.microsoft.com/oryx/builder:debian-buster-20230830.1" myimg

currently fails (dotnet restore hangs / eventually fails).

This is likely due to dotnet not supporting QEMU emulation.

Solution context

In .NET 8 (introduced -a <ARCH> flag), there should be a path to allow this, which involves using --platform using a Multiplatform-tagged image, and specify -a <ARCH> (.NET 8) or -rid (.NET 7):

• .NET 8 blogpost: https://devblogs.microsoft.com/dotnet/improving-multiplatform-container-support/
• Working example: https://gist.github.com/ptupitsyn/1c0f4d6186f63c0f09689745242ebee5