Fix git pull error in core contrib test (open-telemetry#3357)

* Fix git pull error in core contrib test

Core contrib is intermittently failing when
pulling the core repo because tox does not
retry on failure.

Add gh actions/checkout for the core repo
before running tox to mitigate this.

* generate-workflows

Signed-off-by: emdneto <[email protected]>

* Update CHANGELOG.md

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Signed-off-by: emdneto <[email protected]>
Co-authored-by: emdneto <[email protected]>
Co-authored-by: Leighton Chen <[email protected]>

Author: 3 people