“xcrun altool” not found on macOS

I needed to attempt a notarization of a Mac app (currently still in the middle of that long story), but I wasn’t using Xcode, so I had to run tools manually. Unfortunately, I couldn’t use xcrun altool, as it couldn’t find the command. I had upgraded the version of Xcode I was using, so I guess I had some stale links somewhere. Running xcode-select -r did the trick and fixed it.