ci: fix the jobname of the GETTEXT_POISON job

In 6cdccfce1e (i18n: make GETTEXT_POISON a runtime option,
2018-11-08), the `jobname` was adjusted to have the `GIT_TEST_` prefix,
but that prefix makes no sense in this context.

Co-authored-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>

.travis.yml

@@ -16,7 +16,7 @@ compiler:
 
 matrix:
   include:
-    - env: jobname=GIT_TEST_GETTEXT_POISON
+    - env: jobname=GETTEXT_POISON
       os: linux
       compiler:
       addons:

ci/lib.sh

@@ -211,7 +211,7 @@ osx-clang|osx-gcc)
 	# Travis CI OS X
 	export GIT_SKIP_TESTS="t9810 t9816"
 	;;
-GIT_TEST_GETTEXT_POISON)
+GETTEXT_POISON)
 	export GIT_TEST_GETTEXT_POISON=true
 	;;
 Linux32)