macOS Homebrew CI failing

[traversaro]

Since 3 days ago (last successful build is https://github.com/robotology/robotology-superbuild/actions/runs/429511263) the macOS Homebrew build is failing with:

2020-12-21T02:37:53.3978400Z �[32m==>�[0m �[1mInstalling glib dependency: �[32mpython@3.9�[39m�[0m
2020-12-21T02:37:53.3979430Z �[34m==>�[0m �[1mPouring python@3.9-3.9.1_1.catalina.bottle.tar.gz�[0m
2020-12-21T02:37:59.5404030Z �[31mError:�[0m The `brew link` step did not complete successfully
2020-12-21T02:38:00.2855040Z The formula built, but is not symlinked into /usr/local
2020-12-21T02:38:00.2855590Z Could not symlink bin/2to3
2020-12-21T02:38:00.2856080Z Target /usr/local/bin/2to3
2020-12-21T02:38:00.2856530Z already exists. You may want to remove it:
2020-12-21T02:38:00.2857700Z   rm '/usr/local/bin/2to3'
2020-12-21T02:38:00.2858370Z 
2020-12-21T02:38:00.2858830Z To force the link and overwrite all conflicting files:
2020-12-21T02:38:00.2859760Z   brew link --overwrite python@3.9
2020-12-21T02:38:00.2860080Z 
2020-12-21T02:38:00.2860450Z To list all files that would be deleted:
2020-12-21T02:38:00.2861310Z   brew link --overwrite --dry-run python@3.9
2020-12-21T02:38:00.2861650Z 
2020-12-21T02:38:00.2862410Z Possible conflicting files are:
2020-12-21T02:38:00.2863770Z /usr/local/bin/2to3 -> /Library/Frameworks/Python.framework/Versions/2.7/bin/2to3

Fortunately we now have the conda-forge-based CI, so we should catch macOS regressions even if we do not immediately fix this.

[traversaro]

Upstream issue: actions/runner-images#2322 .