Generate a locale-agnostic initdb template
Fixup for
252dcb32397f.
Without this, the "template" data directory created in the initial test
steps uses a non-C locale, upsetting numerous tests that rely on parsing
English error messages.
Discussion: https://postgr.es/m/CAFj8pRB=XVWC0orWu0FbjrmyOpAMLqJiau80YyQOYQPfMj8Xxw@mail.gmail.com