pg_regress: for --no-locale, use LOCALE='C'.
Instead of specifying LC_COLLATE='C' and LC_CTYPE='C', specify
LOCALE='C' which will also affect ICU. This makes pg_regress
consistent with recent changes to initdb in commit
a14e75eb0b6.
Fixes buildfarm failure.
Discussion: https://postgr.es/m/
2458565.
1686953169@sss.pgh.pa.us