Fix ancient memory leak in contrib/auto_explain.

The ExecutorEnd hook is invoked in a context that could be quite
long-lived, not the executor's own per-query context as I think
we were sort of assuming.  Thus, any cruft generated while producing
the EXPLAIN output could accumulate over multiple queries.  This can
result in spectacular leakage if log_nested_statements is on, and
even without that I'm surprised nobody complained before.

To fix, just switch into the executor's context so that anything we
allocate will be released when standard_ExecutorEnd frees the executor
state.  We might as well nuke the code's retail pfree of the explain
output string, too; that's laughably inadequate to the need.

Japin Li, per report from Jeff Janes.  This bug is old, so
back-patch to all supported branches.

Discussion: https://postgr.es/m/CAMkU=1wCVtbeRn0s9gt12KwQ7PLXovbpM8eg25SYocKW3BT4hg@mail.gmail.com

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