Specify locale provider for pg_regress --no-locale

Specify locale provider for pg_regress --no-locale

pg_regress has a --no-locale option that forces the temporary database to
have C locale.  However, currently, locale C only exists in the 'builtin'
locale provider.  This makes 'pg_regress --no-locale' fail when the default
locale provider is not 'builtin'.  This commit makes 'pg_regress --no-locale'
specify both LOCALE='C' and LOCALE_PROVIDER='builtin'.

Discussion: https://postgr.es/m/b54921f95e23b4391b1613e9053a3d58%40postgrespro.ru
Author: Oleg Tselebrovskiy <o.tselebrovskiy@postgrespro.ru>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>