shippable: use C locale to simplify console output

remove this noise:

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
    LANGUAGE = (unset),
    LC_ALL = "en_US.UTF-8",
    LC_CTYPE = "en_US.UTF-8",
    LANG = "en_US.UTF-8"
    are supported and installed on your system.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.shippable.yml

@@ -1,5 +1,7 @@
 language: c
 env:
+  global:
+    - LC_ALL=C
   matrix:
     - IMAGE=debian-armhf-cross
       TARGET_LIST=arm-softmmu,arm-linux-user