Can't upgrade macports under Big Sur! I just upgraded to big Sur and the macports upgrade instructions don't work.
I keep getting this error:
Warning: The macOS 11.0 SDK does not appear to be installed. Ports may not build correctly.
Warning: You can install it as part of the Xcode Command Line Tools package by running `xcode-select --install'.

But I do have Xcode Command Line Tools installed:
% xcode-select --install
xcode-select: error: command line tools are already installed, use &quot;Software Update&quot; to install updates
%

What's up?
It turns out that the Big Sur's XCode command-line tools aren't good enough for MacPorts. You need to manually download Command Line Tools for Xcode 12.2 from https://developer.apple.com/download/more/?=command%20line%20tools