Unexport CIRRUS_COMMIT_MESSAGE

Cirrus CI sets this environment variable from the pull request
description.  However it does not seem any locales other than C
are installed, the default external encoding is US-ASCII.  That
means this environment variable will be an invalid byte sequence,
and some tests fail.

.cirrus.yml

@@ -58,5 +58,7 @@ task:
   make_install_script: make install
   install_gems_for_test_script: $RUBY_PREFIX/bin/gem install --no-doc timezone tzinfo
   make_test-tool_script: make test-tool
-  make_test-all_script: make test-all
+  make_test-all_script:
+    - export -n CIRRUS_COMMIT_MESSAGE
+    - make test-all
   make_test-spec_script: make test-spec