Fix pin image tests for backport MRs

What does this MR do?

The pin image specs testing master behaviour fail in backport pipelines. Fixed by unsetting the target branch environment variable before all specs.

• Failing backport pipeline: https://gitlab.com/gitlab-org/charts/gitlab/-/pipelines/1201741678
• Fixed backport pipeline: https://gitlab.com/gitlab-org/charts/gitlab/-/pipelines/1202801228

Related issues

Relates #5111 (closed)

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