The //flutter/build/archives:artifacts target is used to build a zip archive (artifacts.zip) of host tools such as flutter_tester, the Dart kernel compiler, the impellerc shader compiler, and other tooling that is bundled in debug-mode host builds.

This moves the !is_android to the definition of build_engine_artifacts. This is required because of the way that we produce 32-bit arm gen_snapshot for Android on Windows hosts, which relies on the regular x64 host toolchain due to us having no 32-bit arm toolchain for Windows. As such, current_toolchain == host_toolchain on that platform.

build_engine_artifacts =
    flutter_build_engine_artifacts &&
    (current_toolchain == host_toolchain ||
     (is_linux && !is_chromeos && current_cpu != "arm") || is_mac || is_win)

On iOS builds, we don't have this issue since current_toolchain will be one of:

• //build/toolchain/mac:ios_clang_arm
• //build/toolchain/mac:ios_clang_arm_sim
• //build/toolchain/mac:ios_clang_x64_sim

Whereas host_toolchain will be one of:

• //build/toolchain/mac:clang_arm64
• //build/toolchain/mac:clang_x64

This patch also adds documentation to clarify the purpose of this target and where related artifacts are produced so that future readers don't need to do a deep dive into our build plumbing to figure this out.

While the target itself is primarily intended for producing host binaries, one target binary (gen_snapshot) is bundled into the same archive bundle as the host tools. This should be refactored such that just like iOS and Android, they are bundled into their own target-platform-specific archive, and the tool code accordingly updated to pull these down into the appropriate cache directory.

As a side-note, on macOS we do rely on this archive target for the host tools, but the bundled gen_snapshot is unused -- instead, one produced by the //flutter/sky/tools/create_macos_gen_snapshots.py script used. This should be fixe in a followup patch.

Related: flutter/flutter#38935

Identified while trying to resolve:
Issue: flutter/flutter#101138
Issue: flutter/flutter#69157

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