Fix update-vendor.sh logging

[thockin]

Previously it would corrupt the log when it ran stuff like:
go mod tidy >> "${LOG_FILE}" 2>&1
because this would reopen the file. Also, if that failed, the finish function would be called ALSO with output to the log.

Now we let &1 and &2 always be the log, and &11 and &22 are the real stdout/stderr, which means we have to say that explicitly when we want output.

No, I cannot do OUT="&11" - I would have to use eval to make that work.

/kind bug
/kind cleanup

NONE

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[liggitt]

a couple questions, but seems fine... ran before/after locally and normal output looked identical and normal log content looked ~identical

[thockin]

Re-pushed. The failure I was chasing now logs properly.

[liggitt]

/sig architecture
/area code-organization
/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 1a0ac67de3af920d389322d009ad8ad13e0956c0

[k8s-ci-robot]

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