xwin.Dockerfile: Include `winrt` headers in MSVC "sysroot" search path #303

Job	Run time
cargo-deny	23s
Verify deterministic	2m 38s
Verify CLI	52s
Verify compiles	2m 49s
Lint (ubuntu-22.04)	1m 35s
Publish Check (x86_64-unknown-linux-musl)	56s
Publish Check (aarch64-unknown-linux-musl)	54s
Release	0s
	10m 7s

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