refactor:[M3-9877][SECURITY] - Fix excessive secrets exposure warning in e2e GHA workflow

[bill-akamai]

Description 📝

This PR fixes the Excessive Secrets Exposure vulnerability in our E2E GitHub Actions workflow to stop using dynamic secret lookups and instead use env variables scoped to each matrix job.

Changes 🔄

• Replaced dynamic secret access (secrets[matrix.user.name]) with env-based access (env[matrix.user.name]) for MANAGER_OAUTH in .github/workflows/e2e_schedule_and_push.yml

Scope 🚢

Upon production release, changes in this PR will be visible to:

• All customers
• Some customers (e.g. in Beta or Limited Availability)
• No customers / Not applicable

Verification steps

Full verification will need to be done after merging. In the meantime, I was able to partially verify in my forked repo by adding a copy of the .github/workflows/e2e_schedule_and_push.yml file with my change as this debug Action in GitHub. I added secrets for USER_1 and USER_2 in my repo and could see that they're logging a masked value with the new format of env[matrix.user.name]. I ran my repo's CodeQL Action and saw it only logs an Excessive Secrets Exposure issue for the existing .github/workflows/e2e_schedule_and_push.yml and not the new debug version as expected. IRL, we can verify post-merge as follows:

• After this PR is merged to develop, check for successful run on Tests On Push/Schedule GitHub action
• Confirm CodeQL completes without Excessive Secrets Exposure warnings for this file in the Security/Code scanning area in GitHub: https://github.com/linode/manager/security/code-scanning

Author Checklists

As an Author, to speed up the review process, I considered 🤔

👀 Doing a self review
❔ Our contribution guidelines
🤏 Splitting feature into small PRs
➕ Adding a changeset
🧪 Providing/improving test coverage
🔐 Removing all sensitive information from the code and PR description
🚩 Using a feature flag to protect the release
👣 Providing comprehensive reproduction steps
📑 Providing or updating our documentation
🕛 Scheduling a pair reviewing session
📱 Providing mobile support
♿ Providing accessibility support

• I have read and considered all applicable items listed above.

As an Author, before moving this PR from Draft to Open, I confirmed ✅

• All tests and CI checks are passing
• TypeScript compilation succeeded without errors
• Code passes all linting rules

[linode-gh-bot]

Cloud Manager UI test results

🔺 2 failing tests on test run #2 ↗︎

❌ Failing	✅ Passing	↪️ Skipped	🕐 Duration
2 Failing	703 Passing	4 Skipped	127m 11s

Details

Failing Tests
	Spec	Test
❌	linode-storage.spec.ts	Cloud Manager Cypress Tests→linode storage tab » delete disk
❌	restricted-user-details-pages.spec.ts	Cloud Manager Cypress Tests→restricted user details pages » should disable action elements and buttons in the 'Linodes' details page

Troubleshooting

Use this command to re-run the failing tests:

pnpm cy:run -s "cypress/e2e/core/linodes/linode-storage.spec.ts,cypress/e2e/core/account/restricted-user-details-pages.spec.ts"

[bnussman-akamai]

Will verify once merged!