Switches to version 4 of the checkout GitHub action

Version 1 of the `checkout` GitHub action is supposedly triggering
an issue in Coveralls, which causes the build to fail sporadically.

For more details on the issue, see lemurheavy/coveralls-public#1595

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
         os: [windows-latest, ubuntu-latest]
     steps:
       - name: Get the sources
-        uses: actions/checkout@v1
+        uses: actions/checkout@v4
       - name: Install Node 20
         uses: actions/setup-node@v4
         with:

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
       tools-directory: integrationtests/script/tools
     steps:
       - name: Get the sources
-        uses: actions/checkout@v1
+        uses: actions/checkout@v4
       - name: Install Node 20
         uses: actions/setup-node@v4
         with:

README.md

@@ -150,7 +150,7 @@ jobs:
         os: [windows-latest, ubuntu-latest, macOS-latest]
     steps:
       - name: Get the sources
-        uses: actions/checkout@v1
+        uses: actions/checkout@v4
       - name: Run the build script
         uses: cake-build/cake-action@v2
         with: