fix: pass MCPP_VENDORED_XLINGS in Linux CI build step

On cache miss (new mcpp.toml version), the sandbox lacks the xlings
binary. MCPP_VENDORED_XLINGS tells mcpp where to find it.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -90,6 +90,7 @@ jobs:
 
       - name: Build mcpp from source (self-host)
         run: |
+          export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
           "$MCPP" build
 
       - name: Unit + integration tests via `mcpp test`