[Do not merge] Specify Accept header for toolcache.downloadTool

[mbg]

For testing against main only.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.
• High risk: Changes are not fully under feature flags, have limited visibility and/or cannot be tested outside of production.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.