ci: add minimum GitHub token permissions for workflow

[ashishkurmi]

Description

This PR adds minimum token permissions for the GITHUB_TOKEN in GitHub Actions workflows using secure-workflows.

The GitHub Actions workflow has a GITHUB_TOKEN with write access to multiple scopes. Here is an example of the permissions in one of the workflow runs:
https://github.com/tensorflow/probability/actions/runs/3154612754/jobs/5132400045#step:1:19

After this change, the scopes will be reduced to the minimum needed for the following workflow:

• continuous-integration.yml

Motivation and Context

• This is a security best practice, so if the GITHUB_TOKEN is compromised due to a vulnerability or compromised Action, the damage will be reduced.
• GitHub recommends defining minimum GITHUB_TOKEN permissions.
• The Open Source Security Foundation (OpenSSF) Scorecards also treats not setting token permissions as a high-risk issue. This change will help increase the Scorecard score for this repository.

Signed-off-by: Ashish Kurmi akurmi@stepsecurity.io

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[ashishkurmi]

Have created #1630 to complete the CLA step