Update CI builds to Debian Bullseye, fix associated compatibility bugs

This is a change intended for 0.4.7 maintenance as well as main.

The CI builds use Debian Buster which is now end of life, and I was
experiencing inconsistent CI failures with accessing its security update
server. I wanted to update CI to a distro that isn't EOL, and Bullseye
is the current stable release of Debian.

This opened up a small can of worms that this commit also deals with.
In particular there's a docker engine bug that we work around by
removing the docker-specific apt cleanup script if it exists, and
there's a new incompatibility between tracing and sandbox support.

The tracing/sandbox incompatibility itself had two parts:

  - The membarrier() syscall is used to deliver inter-processor
    synchronization events, and the external "userspace-rcu"
    data structure library would make assumptions that if membarrier
    is available at initialization it always will be. This caused
    segfaults in some cases when running trace + sandbox. Resolved this
    by allowing membarrier entirely, in the sandbox.

  - userspace-rcu also assumes it can block signals, and fails
    hard if this can't be done. We already include a similar carveout
    to allow this in the sandbox for fragile-hardening, so I extended
    that to cover tracing as well.

Addresses issue #40799

Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>

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