Fix the luna tests by downgrading conan (#3151)

toolset/setup/linux/systools/conan.sh

@@ -4,7 +4,10 @@ fw_depends python-dev
 
 fw_installed conan && return 0
 
-pip install --user conan
+# We're using conan 0.28.1 because, as of this writing, later versions of conan
+# are incompatible with the luna framework, and luna is the only framework that
+# uses conan.
+pip install --user -Iv conan==0.28.1
 
 echo -e "export PATH=~/.local/bin:\$PATH" > $IROOT/conan.installed