Add manual DCO check using Python script dco-check (#5528)

## Which problem is this PR solving?
- Several PRs are stuck with DCO check pending. Apparently there's an
ongoing issue with the DCO app: dcoapp/app#211

## Description of the changes
- Add an alternative DCO check into the lint workflow using
https://github.com/christophebedard/dco-check

## How was this change tested?
Ran the script manually:
```shell
$ python3 scripts/dco_check.py -b main
Detected: git (default)

Checking commits: 5863430..7a5e58b

All good!
```

With an unsigned commit:
```shell
$ gca -m bad
[manual-dco-check 7da95684] bad
 1 file changed, 1 insertion(+)

$ python3 scripts/dco_check.py -b main
Detected: git (default)

Checking commits: 5863430..7da95684408ebd3f32692e9444940bdf0e04348d

Missing sign-off(s):

	7da95684408ebd3f32692e9444940bdf0e04348d
		no sign-off found
```

---------

Signed-off-by: Yuri Shkuro <[email protected]>

.github/workflows/ci-lint-checks.yaml

@@ -41,3 +41,13 @@ jobs:
 
     - name: Ensure PR is not on main branch
       uses: ./.github/actions/block-pr-not-on-main
+
+    - name: Set up Python 3.x for DCO check
+      uses: actions/setup-python@v2
+      with:
+        python-version: '3.x'
+
+    - name: Run DCO check
+      run: python3 scripts/dco_check.py -b main -v
+      env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}