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Avoid sed bundled with Darwin for building test images
Change-Id: I4fc1d495c9a42d081107829cfae0cad6d5aa29ff
Signed-off-by: Davanum Srinivas <davanum@gmail.com>
**What this PR does / why we need it**:
using plain old `sed` we run into problems on macOS. for example when you try `make all-container WHAT=net` under test/images. We should use the same pattern we use in all our scripts to ensure we use gsed or a version of sed from GNU.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #
**Special notes for your reviewer**:
/cc @mkumatag @BenTheElder @cjwagner
**Release note**:
```release-note
NONE
```