Add extra GitHub CI check for no_std build.

.github/workflows/extra-check.yml

@@ -0,0 +1,21 @@
+name: extra-check
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  # https://github.com/cspr-rad/kairos/pull/61#issuecomment-2035637388
+  build-kairos-tx-without-std:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - name: Build no_std version of `kairos-tx`
+        run: cargo build -p kairos-tx --no-default-features