GitHub CI: don't run tests with unsupported locales (re: 4a24320)

Now that ksh initialises the locale correctly, the nl_NL.UTF-8
tests are failing on the CI with 'unsupported locale' messages. An
experiment on a test branch revealed that the GitHub CI runners
don't have support for non-USA locales installed at all. The output
of 'locale -a' on them is very short:

    C
    C.utf8
    POSIX
    en_US.utf8

.github/workflows/ci.yml:
- Replace the invalid nl_NL.UTF-8 and ja_JP.SJIS test runs with an
  en_US.utf8 run.

.github/workflows/ci.yml

@@ -21,8 +21,7 @@ jobs:
         : default regression tests &&
         script -q -e -c "bin/package test" &&
         : regression tests with OS-provided multibyte locales &&
-        LANG=nl_NL.UTF-8 script -q -e -c "bin/shtests --locale --nocompile" &&
-        LANG=ja_JP.SJIS  script -q -e -c "bin/shtests --locale --nocompile" &&
+        LANG=en_US.utf8 script -q -e -c "bin/shtests --locale --nocompile" &&
         : disable most SHOPTs, rebuild ksh &&
         sed --regexp-extended --in-place=.orig \
           '/^SHOPT (AUDIT|BGX|BRACEPAT|DEVFD|DYNAMIC|EDPREDICT|ESH|FIXEDARRAY|HISTEXPAND|MULTIBYTE|NAMESPACE|OPTIMIZE|SPAWN|STATS|SUID_EXEC|VSH)=/ s/=1?/=0/' \