Add weekly dependabot for github actions

[sk1p]

Contributor Checklist:

• [ ] I have added or updated my entry in the creators.json file
• [ ] I have added a changelog entry for my contribution
• [ ] I have added/updated documentation for all user-facing changes
• [ ] I have added/updated test cases

Reviewer Checklist:

• [ ] /azp run libertem.libertem-live-data passed

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b9c2293) 74.75% compared to head (f2230f1) 71.11%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #141 +/- ## ========================================== - Coverage 74.75% 71.11% -3.65% ========================================== Files 35 35 Lines 3363 3441 +78 Branches 415 581 +166 ========================================== - Hits 2514 2447 -67 - Misses 713 841 +128 - Partials 136 153 +17 ```

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