test165: set LC_ALL=en_US.UTF-8 too

On my current Debian Unstable with libidn2 2.2.0, I get an error if
LC_ALL is set to blank. Then curl errors out with:

curl: (3) Failed to convert www.åäö.se to ACE; could not convert string to UTF-8

Closes #4738

tests/data/test165

@@ -32,7 +32,7 @@ idn
 proxy
 </features>
 <setenv>
-LC_ALL=
+LC_ALL=en_US.UTF-8
 LC_CTYPE=en_US.UTF-8
 </setenv>
 <precheck>