fix: Ensure TBB is optional using OPTIONAL_COMPONENTS

[phated]

Description

When compiling bb on Mac M1, the cmake configure failed due to TBB not being available. It looks like this is meant to be optional if not available but the find_package call marks it as REQUIRED. It's not enough to just remove REQUIRED because it still assumes the tbb component to be mandatory, so I moved them to OPTIONAL_COMPONENTS to move past this section without failing.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR specifications in /markdown/specs have been updated.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• I've updated any terraform that needs updating (e.g. environment variables) for deployment.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.

[dbanks12]

Looks good.