TUN-6984: [CI] Don't fail on unset.

Dont fail on bash unset (set -u) because we initialise to machine
defaults if the variables are unset within this script.

Author: sudarshan-reddy

.teamcity/build-macos.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-set -euxo pipefail
+set -exo pipefail
 
 if [[ "$(uname)" != "Darwin" ]] ; then
     echo "This should be run on macOS"