Use GHA id-token for sccache-dist auth token

[trxcllnt]

Use the GitHub OIDC token for sccache-dist auth instead of the org's secrets.GIST_REPO_READ_ORG_GITHUB_TOKEN personal access token.

This change is already implemented, this PR just removes any references to GIST_REPO_READ_ORG_GITHUB_TOKEN.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Chores
  • Streamlined GitHub Actions workflows by removing unused secret configurations from build and test pipelines
  • Optimized test matrix configurations to improve workflow execution efficiency

Walkthrough

Three GitHub Actions workflow files are modified to remove sccache distribution token secret references from multiple job configurations. Additionally, one workflow removes RAPIDS auxiliary secret references and updates matrix configurations for test dependencies.

Changes

Cohort / File(s)	Summary
Sccache Token Secret Removals .github/workflows/build.yaml, .github/workflows/pr.yaml, .github/workflows/test.yaml	Removed sccache-dist-token-secret-name configuration entries from job and step definitions across all three workflows (26 removals total).
RAPIDS Auxiliary Secret Removal .github/workflows/pr.yaml	Removed rapids-aux-secret-1 reference and SCCACHE_DIST_AUTH_TOKEN_VAR environment variable from devcontainer configuration.
Matrix Configuration Updates .github/workflows/pr.yaml, .github/workflows/test.yaml	Expanded and restructured matrix configurations—conda-python-scikit-learn-accel-tests now uses combined matrix with dependency branching; conda-python-cuml-accel-upstream-tests extends matrix_filter expression.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: switching sccache-dist authentication from GitHub org secrets to GitHub Actions OIDC id-token.
Description check	✅ Passed	The description is directly related to the changeset, explaining the rationale and scope of removing references to GIST_REPO_READ_ORG_GITHUB_TOKEN in favor of OIDC id-token.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[csadorf]

Blocked by #7788

[trxcllnt]

/merge