os_detect: support macOS Big Sur

[okalachev]

rosdep update is now failing on macOS Big Sur.

This commit makes it not to fail and detect it correctly.

[bjajoh]

@dirk-thomas @tfoote Please merge if possible, I'm also highly interested in this fix. :)

[dirk-thomas]

@bjajoh I am not the maintainer of this repository anymore. Hopefully the new maintainer will provide you with feedback soon (unfortunately I don't recall who that is).

[rtwomey]

I believe this will cover Big Sur through the beta, but the current latest version is 11.1. As of this writing, running sw_vers -productVersion unfortunately returns 11.1. I guess that means "Big Sur" is the codename for both 11.0 and 11.1....

[smnogar]

I believe this will cover Big Sur through the beta, but the current latest version is 11.1. As of this writing, running sw_vers -productVersion unfortunately returns 11.1. I guess that means "Big Sur" is the codename for both 11.0 and 11.1....

I just pushed a new PR that should address this issue.

[tfoote]

Closing in favor of #216