CI: pre-install Rust + iOS targets to fix rustup-during-podbuild race

Build was failing with:
  component download failed for rust-std-aarch64-apple-ios-sim:
  could not rename downloaded file from '.../9362b66...partial' to
  '.../9362b66...': No such file or directory

One of the iOS pods (likely reown_walletkit's yttrium FFI or similar)
invokes rustup mid-build to fetch iOS targets. When the pod build runs
multiple targets in parallel, multiple rustup processes race on the
same .partial download file — one finishes/renames, the next looks
for the .partial that's no longer there.

Fix: add a workflow step that runs before pod install + flutter build:
  - Install rustup if missing (idempotent — self-hosted runner persists).
  - Wipe any stale *.partial files (debris from previous racing).
  - rustup target add the three iOS triples serially.

That way the pod's mid-build rustup invocation finds everything already
installed and just no-ops.

.github/workflows/build-ios-sim.yml

@@ -75,6 +75,24 @@ jobs:
           fi
           pod --version
 
+      - name: Install Rust + iOS targets (if missing)
+        run: |
+          if ! command -v rustup >/dev/null; then
+            curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --no-modify-path
+            echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+            export PATH="$HOME/.cargo/bin:$PATH"
+          fi
+          # Clean any partial downloads from a previously crashed install — those
+          # are the source of the 'could not rename .partial → final: No such
+          # file or directory' race that breaks parallel pod builds.
+          rm -rf "$HOME/.rustup/downloads"/*.partial 2>/dev/null || true
+          # Pre-install iOS targets serially so the pod build doesn't try to
+          # do it under concurrency.
+          rustup target add aarch64-apple-ios
+          rustup target add aarch64-apple-ios-sim
+          rustup target add x86_64-apple-ios
+          rustup show
+
       - name: Install Go + gomobile (if missing)
         run: |
           if ! command -v go >/dev/null; then

.github/workflows/build-ios-testflight.yml

@@ -79,6 +79,19 @@ jobs:
           fi
           pod --version
 
+      - name: Install Rust + iOS targets (if missing)
+        run: |
+          if ! command -v rustup >/dev/null; then
+            curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --no-modify-path
+            echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+            export PATH="$HOME/.cargo/bin:$PATH"
+          fi
+          rm -rf "$HOME/.rustup/downloads"/*.partial 2>/dev/null || true
+          rustup target add aarch64-apple-ios
+          rustup target add aarch64-apple-ios-sim
+          rustup target add x86_64-apple-ios
+          rustup show
+
       - name: Install Go + gomobile (if missing)
         run: |
           if ! command -v go >/dev/null; then