ci: Add a nightly_channel job

This job is in preparation for switching most of the CI to operating on the
stable channel. We still need some actions to run on the nightly channel so we
can test the `unstable` feature and run tests under miri.

To that end, move various steps under a single `nightly_channel` job. The job
switches to the nightly channel using a new `nightly_toolchain.toml` file, so
once we switch the top-level `rust-toolchain.toml` to stable, the new job will
remain on nightly.

.github/workflows/nightly_toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "nightly"
+targets = ["aarch64-unknown-uefi", "i686-unknown-uefi", "x86_64-unknown-uefi"]

.github/workflows/rust.yml

@@ -71,11 +71,6 @@ jobs:
       - name: Run cargo test (without unstable)
         run: cargo xtask test
 
-      # At least one unit test, for make_boxed() currently, has different behaviour dependent on
-      # the unstable feature.
-      - name: Run cargo test (with unstable)
-        run: cargo xtask test --unstable
-
   lints:
     name: Lints
     runs-on: ubuntu-latest
@@ -96,24 +91,9 @@ jobs:
       - name: Run cargo doc (without unstable)
         run: cargo xtask doc --warnings-as-errors --document-private-items
 
-      - name: Run cargo doc (with unstable)
-        run: cargo xtask doc --warnings-as-errors --document-private-items --unstable
-
       - name: Verify generated code is up-to-date
         run: cargo xtask gen-code --check
 
-  miri:
-    name: Run unit tests and doctests under Miri
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout sources
-        uses: actions/checkout@v3
-
-      - name: Run miri
-        run: |
-          rustup component add miri
-          cargo xtask miri
-
   # This job tests that the template app builds successfully with the
   # released versions of the libraries on crates.io.
   #
@@ -173,8 +153,8 @@ jobs:
     - name: Build
       run: cargo xtask build --feature-permutations
 
-  vm_test_unstable:
-    name: Run uefi-test-runner with `unstable` on x86_64
+  nightly_channel:
+    name: Run checks that require the nightly channel
     runs-on: ubuntu-latest
     steps:
     - name: Checkout sources
@@ -185,6 +165,20 @@ jobs:
         sudo apt-get update
         sudo apt-get install qemu-system-x86 ovmf -y
 
-    - name: Run VM tests
+    - name: Enable nightly toolchain
+      run: cp .github/workflows/nightly_toolchain.toml rust-toolchain.toml
+
+    - name: Run VM tests with the `unstable` feature
       run: cargo xtask run --target x86_64 --headless --ci --unstable
       timeout-minutes: 4
+
+    - name: Run cargo doc with the `unstable` feature
+      run: cargo xtask doc --warnings-as-errors --document-private-items --unstable
+
+    - name: Run tests with the `unstable` feature
+      run: cargo xtask test --unstable
+
+    - name: Run unit tests and doctests under Miri
+      run: |
+        rustup component add miri
+        cargo xtask miri