Add check for aarch64 on QEmu

[sindre-nistad]

I discovered that shellingham did not detect the correct shell when running in Docker on a x64 machine while emulating linux/arm64.

Through some debugging, and inspecting the running code, I saw the name variable in shellingham/posix/__init__.py:92 was qemu-aarch64.
Adding that to the regex solved the problem.

I hope you find this useful, and thanks for a good package 😊