Add CI workflow for verifying no_std builds of pczt and `zcash_ke…

…ys`.

.github/workflows/ci.yml

@@ -288,6 +288,46 @@ jobs:
         working-directory: ./ci-build
         run: cargo build --verbose --target ${{ matrix.target }}
 
+  build-nostd:
+    name: Build target ${{ matrix.target }}
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        target:
+          - thumbv7em-none-eabihf
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          path: crates
+      # We use a synthetic crate to ensure no dev-dependencies are enabled, which can
+      # be incompatible with some of these targets.
+      - name: Create synthetic crate for testing
+        run: cargo init --lib ci-build
+      - name: Copy Rust version into synthetic crate
+        run: cp crates/rust-toolchain.toml ci-build/
+      - name: Copy patch directives into synthetic crate
+        run: |
+          echo "[patch.crates-io]" >> ./ci-build/Cargo.toml
+          cat ./crates/Cargo.toml | sed "0,/.\+\(patch.crates.\+\)/d" >> ./ci-build/Cargo.toml
+      - name: Add no_std pragma to lib.rs
+        run: |
+          echo "#![no_std]" > ./ci-build/src/lib.rs
+      - name: Add zcash_keys as a dependency of the synthetic crate
+        working-directory: ./ci-build
+        run: cargo add --no-default-features --path ../crates/zcash_keys
+      - name: Add pczt as a dependency of the synthetic crate
+        working-directory: ./ci-build
+        run: cargo add --no-default-features --path ../crates/pczt
+      - name: Add lazy_static with the spin_no_std feature
+        working-directory: ./ci-build
+        run: cargo add lazy_static --features "spin_no_std"
+      - name: Add target
+        working-directory: ./ci-build
+        run: rustup target add ${{ matrix.target }}
+      - name: Build for target
+        working-directory: ./ci-build
+        run: cargo build --verbose --target ${{ matrix.target }}
+
   bitrot:
     name: Bitrot check
     runs-on: ubuntu-latest