doc: add linux/riscv64 valid combination
doc: add linux/riscv64 valid combination

Mention valid combination GOOS=linux and GOARCH=riscv64
in the "Installing Go from source" document.

Updates #27532

Change-Id: I8c1406087132f5c82a2eee5dbcda95d53c64d263
GitHub-Last-Rev: ee74ff9517bd428b75400956d5b7f774ff800ae5
GitHub-Pull-Request: golang/go#41063
Reviewed-on: https://go-review.googlesource.com/c/go/+/250997
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>

nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go