pull-toolchain.sh: don't run update-flake.sh

We no longer carry an SRI hash for the toolchain, so flake
updating is no longer needed for toolchain changes.

Signed-off-by: David Anderson <danderson@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