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.

.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: |