Add aarch64 wheels to build workflow

[MichaIng]

Description:

Would it be feasible to add aarch64 wheels to the build workflow to be uploaded to PyPI? ARM play an increasing role among servers and pre-compiled wheels save users a lot of compile time and build dependencies, since Rust is required now with v1.68.0.

[DMRobertson]

IIRC we produce aarch64 docker images, so I don't see why we wouldn't also produce wheels for that platform.

cibuildwheel apparently supports aarch64, judging by the table at https://cibuildwheel.readthedocs.io/en/stable/#what-does-it-do

I wonder why manylinux musllinux aarch64 doesn't show up in the binaries we upload. Tests are being skipped here due to *musl* but I don't think that should prevent builds?

synapse/.github/workflows/release-artifacts.yml

Lines 120 to 125 in c9b7e97

	- name: Build wheels
	run: python -m cibuildwheel --output-dir wheelhouse
	env:
	# Skip testing for platforms which various libraries don't have wheels
	# for, and so need extra build deps.
	CIBW_TEST_SKIP: pp39-* *i686* *musl* pp37-macosx*

@erikjohnston any ideas here?

[DMRobertson]

I guess cibuildwheel only builds what it can on a given platform. Would probably have to add a matrix entry to

synapse/.github/workflows/release-artifacts.yml

Lines 92 to 104 in c9b7e97

	build-wheels:
	name: Build wheels on ${{ matrix.os }}
	runs-on: ${{ matrix.os }}
	strategy:
	matrix:
	os: [ubuntu-20.04, macos-10.15]
	is_pr:
	- ${{ startsWith(github.ref, 'refs/pull/') }}
	exclude:
	# Don't build macos wheels on PR CI.
	- is_pr: true
	os: "macos-10.15"

and a step below that conditionally runs the rest under QEMU, like here:

synapse/.github/workflows/docker.yml

Lines 18 to 22 in b8fa24b

	- name: Set up QEMU
	id: qemu
	uses: docker/setup-qemu-action@v1
	with:
	platforms: arm64

[MichaIng]

Finally: #14212

That was a hassle with rust-lang/cargo#10583.

• First I tried to pass CARGO_NET_GIT_FETCH_WITH_CLI it via CIBW_ENVIRONMENT and was wondering why suddenly within the CIBW container the Rust compiler couldn't be found, event that Rust was definitely installed.
• In attempt to fix this, I tried to set CIBW_BEFORE_ALL_LINUX and wondered why since then really no Rust was installed anymore.
• Finally I recognised that these both variables, if not set, are internally set to install Rust (via CIBW_BEFORE_ALL) and to add Rust to PATH (via CIBW_ENVIRONMENT). Setting any of both overrides the default completely, there is no way to only append to the default (AFAIK).
• So the only way to pass CARGO_NET_GIT_FETCH_WITH_CLI without breaking or needing to manually add Rust related steps and variables, is CIBW_ENVIRONMENT_PASS_LINUX, which has no internal default.