[ci] Test macOS-11

[yolo2themoon]

related issue = [ci] Support for macOS-11 x86_64 in CI workflows

Build

brew install llvm@11
export CXX=/usr/local/opt/llvm@11/bin/clang++

Install

We need to rename the wheel name to macosx_10_15 because macosx_11_0 is not a supported tag for pip.
[error message]

mv ./dist/taichi-0.8.10-cp37-cp37m-macosx_11_0_x86_64.whl ./dist/taichi-0.8.10-cp37-cp37m-macosx_10_15_x86_64.whl

Test

Tests passed in [action].

.

Machine info

Build & Install in [action]

ProductName:	macOS
ProductVersion:	11.6.2
BuildVersion:	20G314
Hardware:

    Hardware Overview:

      Model Name: Mac
      Model Identifier: VMware7,1
      Processor Name: Unknown
      Processor Speed: 3.33 GHz
      Number of Processors: 1
      Total Number of Cores: 3
      L2 Cache (per Core): 256 KB
      L3 Cache: 12 MB
      Memory: 14 GB
      System Firmware Version: VMW71.00V.13989454.B64.1906190538
      Apple ROM Info: [MS_VM_CERT/SHA1/27d66596a61c48dd3dc7216fd715126e33f59ae7]Welcome to the Virtual Machine
      SMC Version (system): 2.8f0
      Serial Number (system): VMU4VTj2zF8w
      Hardware UUID: 4203018E-580F-C1B5-9525-B745CECA79EB
      Provisioning UDID: 4203018E-580F-C1B5-9525-B745CECA79EB