docs: document that Actions variables are accessible in Dependabot workflows

[gokcedemir]

Why:

Closes: #43950

What's being changed:

Added a bullet point to the "Restrictions when Dependabot triggers events" section documenting that Actions variables (vars context) are accessible in Dependabot-triggered workflows.

Tested in: https://github.com/gokcedemir/dependabot-variables-test

• Workflow log showed Secret source: Dependabot
• vars.TEST_VAR was successfully accessed with its value

Related community discussion: https://github.com/orgs/community/discussions/44088

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
code-security/reference/supply-chain-security/dependabot-on-actions.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

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