Revert "Upgrade to recent nightly channel"

.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 -Z build-std=core,alloc
+        run: cargo build --no-default-features
       - name: Build dev profile
         run: cargo build
       - name: Unittests on host (ubuntu)

rust-toolchain.toml

 [toolchain]
-channel = "nightly-2021-10-20"
+channel = "nightly-2021-09-29"
 components = [
     "rust-src",
     "llvm-tools-preview",