Fix/ci (#16168)

* fix(ci): allow native x86 Rust toolchain on Windows

* docs(ci): explain the rustup 1.29.1 host check

* docs(ci): explain the unverified x64 cross-compilation alternative

.github/workflows/flutter-build.yml

@@ -492,11 +492,20 @@ jobs:
           version: ${{ env.LLVM_VERSION }}
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@e97e2d8cc328f1b50210efc529dca0028893a2d9 # v1
+        shell: bash
-        with:
+        run: |
-          toolchain: nightly-2023-10-13-${{ matrix.job.target }} # must use nightly here, because of abi_thiscall feature required
+          # Sciter's abi_thiscall feature requires nightly Rust.
-          targets: ${{ matrix.job.target }}
+          # Use an i686 host toolchain so build scripts can load the 32-bit LLVM installed above.
-          components: "rustfmt"
+          # Since rustup 1.29.1, i686 toolchains on x64 Windows require --force-non-host
+          # for both installation and default selection, even though WOW64 can run them.
+          # See https://github.com/rust-lang/rustup/pull/4935.
+          # Alternatively, an x64 compiler could use cargo build --target i686-pc-windows-msvc.
+          # This requires 64-bit LLVM for host build scripts and updated packaging paths,
+          # and has not been manually verified for RustDesk.
+          rustup toolchain install nightly-2023-10-13-${{ matrix.job.target }} \
+            --target ${{ matrix.job.target }} --component rustfmt \
+            --profile minimal --no-self-update --force-non-host
+          rustup default nightly-2023-10-13-${{ matrix.job.target }} --force-non-host
 
       - uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
         with: