Add support for relay-parent-offset. (#122)

Recently CI fails regularly on runtimes repo because try-runtime does
not yet support runtimes with RP offset.

Changes:
- Use polkadot-sdk master (sadly stable2512 does not contain rp_offset
field yet)
- Fetch rp offset from runtime and add it to inherent
- Simplify inherent usage

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

Author: skunert

.github/workflows/rust-checks.yaml

@@ -31,6 +31,10 @@ jobs:
         uses: arduino/setup-protoc@v1
         with:
           version: "3.6.1"
+      - name: Install deps for build
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y clang llvm-dev libclang-dev
       - name: cargo doc
         run: RUSTFLAGS="-D warnings" cargo doc --locked
 
@@ -94,6 +98,10 @@ jobs:
         uses: arduino/setup-protoc@v1
         with:
           version: "3.6.1"
+      - name: Install deps for build
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y clang llvm-dev libclang-dev
       - name: Free disk space
         run: |
           sudo docker rmi $(docker image ls -aq) >/dev/null 2>&1 || true