go.dev: turn off share button on golang.google.cn
go.dev: turn off share button on golang.google.cn

Right now golang.google.cn serves a copy of golang.org.
Eventually it will serve a copy of go.dev instead.
Prepare for that by disabling the share button on the go.dev
home page when the request is actually for golang.google.cn.

Change-Id: I2b80d066ceb789cd188f6409694bccfed2a87c19
Reviewed-on: https://go-review.googlesource.com/c/website/+/362956
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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