Rename LAVAMOAT_UPDATE_TOKEN to a more generic name

### What is this about?

We are currently using `LAVAMOAT_UPDATE_TOKEN` in two workflows: [Update LavaMoat policies](https://github.com/MetaMask/metamask-extension/blob/develop/.github/workflows/update-lavamoat-policies.yml) and [Update Attributions](https://github.com/MetaMask/metamask-extension/blob/develop/.github/workflows/update-attributions.yml). This token should be renamed to a more generic name that does not include the LavaMoat keyword.

### Scenario

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### Design

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### Technical Details

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### Threat Modeling Framework

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### Acceptance Criteria

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### Stakeholder review needed before the work gets merged

- [X] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [X] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

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Start by inspecting .github/workflows/update-lavamoat-policies.yml and .github/workflows/update-attributions.yml, then search the repository for LAVAMOAT_UPDATE_TOKEN. Rename the shared token consistently in both workflows and related configuration, with no remaining references to the old name.

Written by the indexing model from the issue text.