Use default plan for CI if not specified.

This allows mmtk-core to do extended test without specifying a plan, so
that mmtk-ruby can decide the default plan without updating mmtk-core.

Author: wks

.github/scripts/common.sh

@@ -3,3 +3,8 @@ RUBY_PATH=$BINDING_PATH/../ruby
 RUBY_BUILD_PATH=$RUBY_PATH/build
 RUBY_INSTALL_PATH=$RUBY_BUILD_PATH/install
 RUSTUP_TOOLCHAIN=`cat $BINDING_PATH/mmtk/rust-toolchain`
+DEFAULT_PLAN=Immix
+
+if test -z "$CHOSEN_PLAN"; then
+    CHOSEN_PLAN=$DEFAULT_PLAN
+fi