Override nightly versions if repo variable set for non scheduled runs (#524)

- [x] no changelog update needed

.github/workflows/build-and-test.yml

@@ -2,49 +2,49 @@ name: Rust
 
 on:
   push:
-    branches: [ master ]
+    branches: [master]
   pull_request:
-    branches: [ master ]
+    branches: [master]
   schedule:
-    - cron: '10 6 * * *'
+    - cron: "10 6 * * *"
 
 env:
   CARGO_TERM_COLOR: always
+  RUSTUP_TOOLCHAIN: ${{ !github.event.schedule && 'nightly-2023-12-01' || 'nightly' }}
 
 jobs:
   build:
     name: Just CI
     runs-on: ubuntu-22.04
     steps:
-    - name: Set CARGO_TARGET_DIR
-      run: echo "CARGO_TARGET_DIR=$HOME/target" >> $GITHUB_ENV
-    - name: Install build tools
-      run: sudo apt-get update && sudo apt-get install build-essential libelf-dev zip -y
-    - name: Install Miri
-      run: |
-          rustup toolchain install nightly --component miri
-          rustup override set nightly
+      - name: Set CARGO_TARGET_DIR
+        run: echo "CARGO_TARGET_DIR=$HOME/target" >> $GITHUB_ENV
+      - name: Install build tools
+        run: sudo apt-get update && sudo apt-get install build-essential libelf-dev zip -y
+      - name: Install Miri
+        run: |
+          rustup component add miri clippy rustfmt
           cargo miri setup
-    - uses: actions/checkout@v4
-    - name: Cache
-      uses: actions/cache@v3
-      with:
-        path: |
-          ~/.cargo/registry
-          ~/.cargo/git
-          ~/target
-        key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
-    - name: install mgba-test-runner
-      run: cargo install --path emulator/test-runner --verbose
-    - uses: extractions/setup-just@v1
-    - name: Setup mdBook
-      uses: peaceiris/actions-mdbook@v1
-      with:
-        mdbook-version: '0.4.13'
-    - name: Build and test all crates
-      run: just ci
-    - name: Upload build artifacts
-      uses: actions/upload-artifact@v3
-      with:
-        name: example-games
-        path: ~/target/*.gba
+      - uses: actions/checkout@v4
+      - name: Cache
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            ~/target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+      - name: install mgba-test-runner
+        run: cargo install --path emulator/test-runner --verbose
+      - uses: extractions/setup-just@v1
+      - name: Setup mdBook
+        uses: peaceiris/actions-mdbook@v1
+        with:
+          mdbook-version: "0.4.13"
+      - name: Build and test all crates
+        run: just ci
+      - name: Upload build artifacts
+        uses: actions/upload-artifact@v3
+        with:
+          name: example-games
+          path: ~/target/*.gba