[develop] Add Big Sur instances to CICD

[scottarnette]

Change Description

Adding support for Big Sur builds in our CICD.

• Skipped by default in all existing pipeline runs while we fix potential issues. Not skipped by default in a new pipeline: https://buildkite.com/EOSIO/eosio-big-sur-beta
• Updated Anka plugin version.
• Dependent VMs not live yet! Should be ready to go by end of week.

See:
https://buildkite.com/EOSIO/eosio-build-unpinned/builds/5584 | Example of Big Sur builds passing.
https://buildkite.com/EOSIO/eosio-big-sur-beta/builds/2 | Example of Big Sur pipeline generation. Does not pass since production Big Sur VMs aren't ready for consumption.

Change Type

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• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

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• New Tests

• Existing Tests

• Test Framework

• CI System

• Other

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions