Weekly Travis build failed on OS X

[lmoureaux]

CMake could not find KF5Archive. Doesn't look like there was an update to the Homebrew bottle recently.

https://travis-ci.org/github/longturn/freeciv21/jobs/766779115

[lmoureaux]

Different log here, CMake cannot find Ninja: https://travis-ci.org/github/longturn/freeciv21/jobs/766814044

[lmoureaux]

Upgrading cmake formula. It is installed but not up-to-date.
==> Upgrading 1 outdated package:
cmake 3.18.4 -> 3.20.1
==> Upgrading cmake 3.18.4 -> 3.20.1 
==> Downloading https://homebrew.bintray.com/cmake-3.20.1.catalina.bottle.tar.gz
curl: (22) The requested URL returned error: 404 Not Found
Trying a mirror...
==> Downloading https://ghcr.io/v2/homebrew/core/cmake-3.20.1.catalina.bottle.tar.gz
==> Downloading from https://github.com/-/v2/packages/container/package/homebrew%2Fcore%2Fcmake-3.20.1.catalina.bottle.tar.gz
curl: (22) The requested URL returned error: 404 
Error: Failed to download resource "cmake"
Download failed: https://ghcr.io/v2/homebrew/core/cmake-3.20.1.catalina.bottle.tar.gz
Installing cmake has failed!

Installing packages from Homebrew fails.

[lmoureaux]

Well, let's use a more recent OS X. We are already using the latest image.