[AIRFLOW-5133] Keep original env state in provide_gcp_credential_file

[mik-laj]

Preserve the state of the environment variable after execution of provide_gcp_credential_file decorator to avoid side-effect.

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
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Code Quality

• Passes flake8

[potiuk]

Needs rebase

[mik-laj]

@potiuk rebased.