Revert "Bump actions/checkout from 3 to 4"

Older versions of Ubuntu (16.04 and 18.04) does not work with this
version. I'll look into this at a later time, but for now just stick
with the old version.

This reverts commit 36aff72.

Author: thus

.github/workflows/alpine.yml

@@ -19,7 +19,7 @@ jobs:
       image: alpine:edge
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v3
 
       - name: Install dependencies
         run: |

.github/workflows/code-analysis.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v3
 
       - name: Install dependencies
         run: |

.github/workflows/conan.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v3
 
       - name: Install Conan
         run: pip install conan

.github/workflows/ubuntu.yml

@@ -26,7 +26,7 @@ jobs:
       image: ubuntu:${{ matrix.release }}
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v3
 
       - name: Install dependencies
         run: |