egl: use the LC_ALL hammer instead of LANG

Some environment (like Travis apparently) set LC_* vars, messing up the
sort ordering, so let's use envvar with the highest priority to make
sure this is actually sorted in ASCII order.

Suggested-by: Michel Dänzer <michel@daenzer.net>
Fixes: b42dc50a5f "egl: fix entrypoint sorting test"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

src/egl/egl-entrypoint-check

@@ -7,5 +7,5 @@ then
 fi
 
 entrypoints=$(grep EGL_ENTRYPOINT "$srcdir"/main/eglentrypoint.h)
-sorted=$(LANG=C sort <<< "$entrypoints")
+sorted=$(LC_ALL=C sort <<< "$entrypoints")
 test "$entrypoints" = "$sorted"