Upgrade toolchain channel to nightly-2021-10-20

CI: Explicitly set build-std without std
Else, cargo unifies with rusty-hermit's outer .cargo/config.toml and
tries to build std for the kernel, which fails with:

error[E0432]: unresolved import `crate::sys::hermit::thread_local_dtor`
 --> /home/mkroening/.rustup/toolchains/nightly-2021-10-20-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/hermit/thread.rs:9:25
  |
9 | use crate::sys::hermit::thread_local_dtor::run_dtors;
  |                         ^^^^^^^^^^^^^^^^^ could not find `thread_local_dtor` in `hermit`

error[E0432]: unresolved import `crate::sys::hermit::thread_local_dtor`
   --> /home/mkroening/.rustup/toolchains/nightly-2021-10-20-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/hermit/mod.rs:122:29
    |
122 |     use crate::sys::hermit::thread_local_dtor::run_dtors;
    |                             ^^^^^^^^^^^^^^^^^ could not find `thread_local_dtor` in `hermit`

.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)

rust-toolchain.toml

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