CI: downgrade Linux runner to Ubuntu 20.04

Fixes failing CI, missing libtinfo5 and potential future issues

Author: Bromeon

.github/workflows/minimal-ci.yml

@@ -88,8 +88,10 @@ jobs:
             rust-toolchain: stable-x86_64-pc-windows-msvc
             godot-binary: godot.windows.editor.dev.x86_64.exe
 
+          # Don't use latest Ubuntu (22.04) as it breaks lots of ecosystem compatibility.
+          # If ever moving to ubuntu-latest, need to manually install libtinfo5 for LLVM.
           - name: linux
-            os: ubuntu-latest
+            os: ubuntu-20.04
             rust-toolchain: stable
             godot-binary: godot.linuxbsd.editor.dev.x86_64