Speed up cross-platform builds

I think we can speed up the Go and Python builds by using buildkit's target platforms.

### Go and Delve

Go supports cross-platform building; I'm uncertain if we can do deep-static linking with `-extldflags "-static"'`.

### Python

`pip` supports `--platform` and `--abi` flags. ​Something like:

​pip install --only-binary=:all: --abi cp39 --platform=manylinux2014_x86_64 --target=/dbg/python/lib/python3.9/site-packages debugpy

The platforms ​and mappings are listed here: https://github.com/pypa/manylinux