Upgrade to actions/checkout@v4 (#731)

## Changes

This should fix intermittent failures with v3 (see
https://github.com/actions/checkout/issues/1448)

.github/workflows/push.yml

@@ -20,7 +20,7 @@ jobs:
 
     steps:
       - name: Checkout repository and submodules
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Unshallow
         run: git fetch --prune --unshallow
@@ -50,7 +50,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Setup Go
         uses: actions/setup-go@v4

.github/workflows/release-snapshot.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository and submodules
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Unshallow
         run: git fetch --prune --unshallow

.github/workflows/release.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository and submodules
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Unshallow
         run: git fetch --prune --unshallow