ci: fix propagating UTF-8 test locale in musl-based Meson job

The musl-based Meson job is supposed to explicitly specify the UTF-8
locale used for testing, which has been introduced with 84bb5ee (ci:
switch linux-musl to use Meson, 2025-01-28). That commit had two issues
though:

  - We continue to refer to "linux-musl", even though the job has been
    renamed in the same commit to "linux-musl-meson".

  - We use the wrong option name to specify the locale. This was not
    noticed though due to the first issue.

Fix both of these issues by fixing both the job and option naems.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

ci/lib.sh

@@ -348,8 +348,8 @@ case "$jobname" in
 linux32)
 	CC=gcc
 	;;
-linux-musl)
-	MESONFLAGS="$MESONFLAGS -DGIT_TEST_UTF8_LOCALE=C.UTF-8"
+linux-musl-meson)
+	MESONFLAGS="$MESONFLAGS -Dtest_utf8_locale=C.UTF-8"
 	;;
 linux-leaks|linux-reftable-leaks)
 	export SANITIZE=leak