po: Use C.UTF-8 locale for Zanata cli
po: Use C.UTF-8 locale for Zanata cli

We don't have en_US.UTF-8 locale on our task bot and it breaks
translations.

Fixes #12856
Closes #12867

deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,deryni/cockpit,cockpituous/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,cockpituous/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,garrett/cockpit