elevate GITHUB_TOKEN to packages feed credential

[brettfo]

When a dependabot job is run, the auto-generated environment variable GITHUB_TOKEN already has the packges: read permissions.

This PR uses that value and elevates it to a GitHub package feed credential if:

1. The experiment automatic_github_packages_auth is enabled
2. The environment variable GITHUB_TOKEN can be resolved
3. The package manager is something that GitHub packages supports
4. No existing duplicate package feed credential exists

[jakecoffman]

Seems right!

[Copilot]

Pull request overview

This PR implements automatic authentication for GitHub Packages by elevating the GITHUB_TOKEN environment variable to a package feed credential when specific conditions are met: the automatic_github_packages_auth experiment is enabled, GITHUB_TOKEN is available, the package manager supports GitHub Packages, and no duplicate credential exists.

Changes:

• Added automatic GitHub Packages credential generation for NuGet when the experiment flag is enabled
• Extended the JobDetails type with a source field containing repository information
• Added comprehensive test coverage for the new credential generation logic

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 5 comments.

File	Description
src/main.ts	Implements getPackagesCredential() and getNuGetPackagesCredential() functions to create GitHub Packages credentials, and integrates them into the main credential workflow
src/api-client.ts	Adds source field to JobDetails type to capture repository information needed for constructing package feed URLs
tests/main.test.ts	Adds comprehensive test suite for getPackagesCredential() covering various scenarios including experiment flags, token availability, and duplicate detection
.gitattributes	Adds binary handling for .exe files (unrelated to main PR purpose)

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