libbacktrace: fix b2test_buildid test on non-english locales

On LANG=ru_RU.UTF-8 'b2test_buildid' test fails due to localized readelf
output:

$ LANG=ru_RU.UTF-8 readelf -n b2test | fgrep 4e37e8f
    ID сборки: 4e37e8fead8d6e8b0a9dc95ea25cd784dff3a393
$ LANG=C readelf -n b2test | fgrep 4e37e8f
    Build ID: 4e37e8fead8d6e8b0a9dc95ea25cd784dff3a393

libbacktrace/

	* install-debuginfo-for-buildid.sh.in: Force non-localized readelf
	output with LANG=C.

install-debuginfo-for-buildid.sh.in

@@ -47,7 +47,7 @@ mkdir_p="@MKDIR_P@"
 build_id_dir="$1"
 src="$2"
 
-buildid=$($readelf -n $src \
+buildid=$(LANG=C $readelf -n $src \
 	      | $grep "Build ID" \
 	      | $awk '{print $3}')