Autovac cancellation is broken in v14

If I create a large table with "CREATE TABLE ... AS SELECT ... from
generate_series(1,3e7)" with no explicit transactions, then once it is done
I wait for autovac to kick in, then when I try to build an index on that
table (or drop the table) the autovac doesn't go away on its own.

Bisects down to:

commit 5788e258bb26495fab65ff3aa486268d1c50b123
Author: Andres Freund <andres@anarazel.de>
Date:   Wed Jul 15 15:35:07 2020 -0700

    snapshot scalability: Move PGXACT->vacuumFlags to
ProcGlobal->vacuumFlags.

Which makes sense given the parts of the code this touches, although I
don't understand exactly what the problem is.  The problem persists in HEAD
(77c7267c37).

Cheers,

Jeff