iOS TestFlight: also install nightly Rust + iOS targets

One of the Flutter plugins in our dep chain (sp_scanner / payjoin /
something in the cargokit family) compiles its rust component with
+nightly. The archive step ran for 27s then bombed with:

  Error (Xcode): Missing manifest in toolchain 'nightly-aarch64-apple-darwin'

We were installing only stable + iOS targets. Add nightly toolchain
(minimal profile) + iOS targets for it alongside the stable ones.
~250 MB extra download once per runner workspace lifetime.

build-ios-sim.yml has the same stable-only setup but never tripped on
this — likely because the failing plugin's release-only build path
isn't exercised in the unsigned sim build. Leaving sim workflow
alone for now; copy the nightly steps over if it ever surfaces there.

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

@@ -90,9 +90,19 @@ jobs:
             export PATH="$HOME/.cargo/bin:$PATH"
           fi
           rm -rf "$HOME/.rustup/downloads"/*.partial 2>/dev/null || true
+          # Stable + iOS targets (most plugins use these)
           rustup target add aarch64-apple-ios
           rustup target add aarch64-apple-ios-sim
           rustup target add x86_64-apple-ios
+          # Nightly + iOS targets — at least one plugin (sp_scanner / payjoin
+          # / something in the cargokit chain) builds its rust component
+          # with +nightly. The TestFlight archive bombs at "Missing manifest
+          # in toolchain 'nightly-aarch64-apple-darwin'" if nightly isn't
+          # installed alongside stable.
+          rustup toolchain install nightly --profile minimal
+          rustup target add aarch64-apple-ios --toolchain nightly
+          rustup target add aarch64-apple-ios-sim --toolchain nightly
+          rustup target add x86_64-apple-ios --toolchain nightly
           rustup show
 
       - name: Install Go + gomobile (if missing)