build: do not build merge commits

.github/workflows/job_build_tr1.yml

@@ -27,6 +27,7 @@ jobs:
         with:
           submodules: 'true'
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha || github.sha }}
 
       - id: vars
         name: Prepare variables

.github/workflows/job_build_tr1_macos.yml

@@ -47,6 +47,7 @@ jobs:
         with:
           submodules: 'true'
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha || github.sha }}
 
       - name: Extend PATH for MacPorts
         run: |

.github/workflows/job_build_tr2.yml

@@ -23,6 +23,7 @@ jobs:
         with:
           submodules: 'true'
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha || github.sha }}
 
       - id: vars
         name: Prepare variables

.github/workflows/job_release.yml

@@ -37,6 +37,7 @@ jobs:
         with:
           submodules: 'true'
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha || github.sha }}
 
       - name: "Prepare release data"
         id: prepare_release_data

.github/workflows/lint.yml

@@ -14,6 +14,7 @@ jobs:
         with:
           submodules: 'true'
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha || github.sha }}
 
       - name: Install dependencies
         run: |