recipe/postgres: use C.UTF-8 locale

en_US.UTF-8 might not be installed and in such case postgres refuses to start.
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

03:44:36

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:02:33