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Extract gnu-sed detection into a function

**What this PR does / why we need it**:
Moves gnu-sed detection into a reusable function across scripts (considering it's in multiple places).

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:
```release-note
NONE
```