tool/gocross, pull-toolchain.sh: support a "next" Go toolchain

When TS_GO_NEXT=1 is set, update/use the
go.toolchain.next.{branch,rev} files instead.

This lets us do test deploys of Go release candidates on some
backends, without affecting all backends.

Updates tailscale/corp#36382

Change-Id: I00dbde87b219b720be5ea142325c4711f101a364
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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