WIP: build aarch64 wheels on release branches

[DMRobertson]

Closes #13938.

Need to check that

• On a PR, we still only build one x64 wheel
  • https://github.com/matrix-org/synapse/actions/runs/3174484886 on 42040d8
• On a release branch, we still build Linux x86 and macOS wheels
• On a release branch, we also build Linux aarch64

[DMRobertson]

Couldn't get this to work. Dropping this.

[MichaIng]

@DMRobertson
The build failed since x86 and aarch64 flags seems to have been mixed. Did you try to replace CIBW_BUILD accordingly?

Suggested change

	echo "CIBW_ARCHS_LINUX=aarch64" >> $GITHUB_ENV
	echo "CIBW_ARCHS_LINUX=aarch64" >> $GITHUB_ENV
	sed -i '/^CIBW_BUILD=/c\CIBW_BUILD=cp37-manylinux_aarch64' $GITHUB_ENV

[MichaIng]

I'm one step further: https://github.com/MichaIng/synapse/pull/1
It failed a lot later: https://github.com/MichaIng/synapse/actions/runs/3259084284/jobs/5351656846

When building the proc-macro2 Rust create it fails, sadly I cannot see the reason, also since verbosity flags are not passed to the Rust compiler. Not sure if `cargo rustc ...` failed with code -9 means that it was SIGKILLed? Since an exit code 9 seems to not exist for cargo and the Rust compiler. Would point into direction of insufficient memory or such, which is quite thinkable for aarch64 Rust builds. I run into memory issues with them in a lot of other cases (emulated and on native hardware as well). Not sure why Rust builds require significantly more memory than x86, armv7l and armv6l builds at least.

This one (completely different software and emulation, just same emulated aarch64) fails at exact same crate while all other archs build fine: https://github.com/MichaIng/DietPi/actions/runs/3259052060/jobs/5351598826
Will try repeating on native aarch64 hardware to see whether there is a general issue.

[DMRobertson]

Not sure if `cargo rustc ...` failed with code -9 means that it was SIGKILLed? Since an exit code 9 seems to not exist for cargo and the Rust compiler. Would point into direction of insufficient memory or such, which is quite thinkable for aarch64 Rust builds.

Likely fixed by #14173---try merging in develop.