go.toolchain.next.rev: update to final Go 1.26.0 [next]

This updates the TS_GO_NEXT=1 (testing) toolchain to Go 1.26.0

The default one is still Go 1.25.x.

Updates #18682

Change-Id: I99747798c166ce162ee9eee74baa9ff6744a62f6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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