Update script doesn't work on arm64 macs

[SimenB]

Running ./update.sh results in functions.sh: line 42: arch: unbound variable. uname -m returns arm64 which is not present in the case, so that makes sense (we really should fix the logging, but whatever).

Adding arm64 to the list (mapping to arm64) generates a diff adding that arch to the Dockerfiles (e.g. FROM arm64/debian:buster-slim), which I don't think is what we want?

We really should land #1368

/cc @nodejs/docker