Remove more obsolete uses of LC_CTYPE

Our test driver unsets all LC_* variables as well as LANGUAGE, and
it sets LANG=C, to make errors show up as

	set_color: Unknown color 'reset'

rather than

	set_color: Unknown color “reset”

It also sets LC_CTYPE which is hardly necessary since c8001b5023
(encoding: use UTF-8 everywhere, 2025-10-18).

The only place where we need it seems to be the use of GNU sed as
called in tests/checks/sphinx-markdown-changelog.fish.

In future we might want to avoid these issues by setting LANG=C.UTF-8
in the test_driver again.

build_tools/release-notes.sh

@@ -62,7 +62,7 @@ previous_minor_version=${previous_version%.*}
 
     if [ "$minor_version" != "$previous_minor_version" ]; then {
         JoinEscaped() {
-            sed 's/\S/\\&/g' |
+            LC_CTYPE=C.UTF-8 sed 's/\S/\\&/g' |
                 awk '
                     NR != 1 { printf ",\n" }
                     { printf "%s", $0 }

tests/test_driver.py

@@ -86,7 +86,6 @@ def makeenv(script_path: Path, home: Path) -> Dict[str, str]:
             "XDG_CACHE_HOME": str(xdg_cache),
             "fish_test_helper": str(home.parent / "fish_test_helper"),
             "LANG": "C",
-            "LC_CTYPE": "en_US.UTF-8",
         }
     )