pull-toolchain.sh: advance the next hash if it's behind

Updates tailscale/corp#36382

Change-Id: Ida55b7b1a2cdd0a4653bb41852008e7088fc4a48
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f