Update to actions/checkout@v4, enable dependabot (#6)

Author: cottsay

.github/dependabot.yaml

@@ -0,0 +1,7 @@
+---
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/build-variants.yaml

@@ -1,3 +1,4 @@
+---
 name: Build Variants
 
 on:
@@ -10,7 +11,7 @@ jobs:
   build-ros-dev-tools:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Earthly
         run: |
           sudo wget 'https://github.com/earthly/earthly/releases/latest/download/earthly-linux-amd64' -O /usr/local/bin/earthly
@@ -22,7 +23,7 @@ jobs:
   build-ros-build-essential:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Earthly
         run: |
           sudo wget 'https://github.com/earthly/earthly/releases/latest/download/earthly-linux-amd64' -O /usr/local/bin/earthly
@@ -35,7 +36,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: [build-ros-build-essential, build-ros-dev-tools]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Earthly
         run: |
           sudo wget 'https://github.com/earthly/earthly/releases/latest/download/earthly-linux-amd64' -O /usr/local/bin/earthly
@@ -46,7 +47,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: [build-ros-build-essential, build-ros-dev-tools]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Earthly
         run: |
           sudo wget 'https://github.com/earthly/earthly/releases/latest/download/earthly-linux-amd64' -O /usr/local/bin/earthly