Ensure cperf sets up custom workspaces correctly

[justice-adams-apple]

Pull Request Description

• Fix Workspace bug in performance jobs after move to update_checkout

  $ git -C /Users/ec2-user/jenkins/workspace-private/swift-PR-compiler-performance-macOS/main/new/swift fetch origin pull/69591/merge
  fatal: cannot change to '/Users/ec2-user/jenkins/workspace-private/swift-PR-compiler-performance-macOS/main/new/swift': No such file or directory
  WORKSPACE: /Users/ec2-user/jenkins/workspace/swift-PR-compiler-performance-macOS
  Traceback (most recent call last):
  File "/Users/ec2-user/jenkins/workspace/swift-PR-compiler-performance-macOS/swift-source-compat-suite/run_cperf", line 632, in <module>
  sys.exit(main())
  File "/Users/ec2-user/jenkins/workspace/swift-PR-compiler-performance-macOS/swift-source-compat-suite/run_cperf", line 117, in main
  setup_workspace(instance, workspace, args)
  File "/Users/ec2-user/jenkins/workspace/swift-PR-compiler-performance-macOS/swift-source-compat-suite/run_cperf", line 54, in setup_workspace
  common.check_execute(command_fetch)
  File "/Users/ec2-user/jenkins/workspace/swift-PR-compiler-performance-macOS/swift-source-compat-suite/common.py", line 220, in check_execute
  raise ExecuteCommandFailure(command, returncode)
  common.ExecuteCommandFailure: ExecuteCommandFailure(command="git -C /Users/ec2-user/jenkins/workspace-private/swift-PR-compiler-performance-macOS/main/new/swift fetch origin pull/69591/merge", returncode=128)