docs: golang plugin doc missing information on cross-compiling

[sed-i]

Please describe the question or issue you're facing with "Go plugin - Snapcraft 8.13.2 documentation".

The "Environment variables" section states that:

During build, this plugin sets GOBIN to ${CRAFT_PART_INSTALL}/bin.

However, when I try to cross-compile on amd64, snapcraft pack errors out with:

Detailed information:
:: + go mod download all
:: + go install -p 8 ./...
:: go: cannot install cross-compiled binaries when GOBIN is set
Recommended resolution: Check the build output and verify the project can work with the 'go' plugin.

But when I clear it,

    build-environment:
      - GOBIN: ""

then pack fails with:

 FileNotFoundError: [Errno 2] No such file or directory: '/root/parts/<snap name>/install/usr'

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Reported from: https://documentation.ubuntu.com/snapcraft/8.13.2/common/craft-parts/reference/plugins/go_plugin/