chore: update CI actions which are triggering warnings (#707)

.github/workflows/build.yml

@@ -21,7 +21,7 @@ jobs:
           ./elan-init -y --default-toolchain none
           echo "$HOME/.elan/bin" >> $GITHUB_PATH
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: build std
         id: build

.github/workflows/nightly_bump_toolchain.yml

@@ -2,7 +2,7 @@ name: Bump lean-toolchain on nightly-testing
 
 on:
   schedule:
-    - cron: '0 9 * * *' 
+    - cron: '0 9 * * *'
     # 10AM CET/1AM PT, 2 hours after lean4 starts building the nightly.
     # Mathlib's `nightly-testing` branch is bumped one hour later.
 
@@ -13,7 +13,7 @@ jobs:
 
     steps:
     - name: Checkout code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
       with:
         ref: nightly-testing # checkout nightly-testing branch
         token: ${{ secrets.NIGHTLY_TESTING }}

.github/workflows/nightly_detect_failure.yml

@@ -34,7 +34,7 @@ jobs:
 
     steps:
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
       with:
         ref: nightly-testing # checkout nightly-testing branch
         fetch-depth: 0 # checkout all branches so that we can push from `nightly-testing` to `nightly-testing-YYYY-MM-DD`

.github/workflows/nightly_merge_master.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
           token: ${{ secrets.NIGHTLY_TESTING }}