CI: add windows-arm runner

[RalfJung]

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[RalfJung]

This doesn't work yet, rustup-toolchain-install-master currently cannot be built on windows-arm since it depends on winapi:

rustup-toolchain-install-master v1.8.1 (/home/r/src/rust/rustup-toolchain-install-master)
├── ansi_term v0.12.1
│   └── winapi v0.3.9
...
├── pbr v1.1.1
│   ├── crossbeam-channel v0.5.15
│   │   └── crossbeam-utils v0.8.21
│   ├── libc v0.2.174
│   └── winapi v0.3.9 (*)

Cc @kennytm
Might be relevant for rust-lang/rfcs#3817