CI: Explicitly set build-std without std

.cargo/config.toml

 [unstable]
+# Keep in sync with CI!
 build-std = ["core", "alloc"]
 build-std-features = ["compiler-builtins-mem"]
 

.github/workflows/x86.yml

@@ -39,7 +39,7 @@ jobs:
         run: rustup show
       - name: Build minimal kernel
         working-directory: libhermit-rs
-        run: cargo build --no-default-features
+        run: cargo build --no-default-features -Z build-std=core,alloc
       - name: Build dev profile
         run: cargo build
       - name: Unittests on host (ubuntu)