[TravisCI] Fix bug. PYTHON_EXECUTABLE should not be in common

defaults. The location is dependent on the implementation.

This triggered a build failure on TravisCI because the location
of the default Python binary is different to what is in the Docker
container.

contrib/ci/Dockerfiles/z3_build.Dockerfile

@@ -12,7 +12,7 @@ ARG DOTNET_BINDINGS
 ARG JAVA_BINDINGS
 ARG NO_SUPPRESS_OUTPUT
 ARG PYTHON_BINDINGS
-ARG PYTHON_EXECUTABLE
+ARG PYTHON_EXECUTABLE=/usr/bin/python2.7
 ARG RUN_SYSTEM_TESTS
 ARG RUN_UNIT_TESTS
 ARG TARGET_ARCH

contrib/ci/scripts/ci_defaults.sh

@@ -9,7 +9,6 @@ export DOTNET_BINDINGS="${DOTNET_BINDINGS:-1}"
 export JAVA_BINDINGS="${JAVA_BINDINGS:-1}"
 export NO_SUPPRESS_OUTPUT="${NO_SUPPRESS_OUTPUT:-0}"
 export PYTHON_BINDINGS="${PYTHON_BINDINGS:-1}"
-export PYTHON_EXECUTABLE="${PYTHON_EXECUTABLE:-$(which python)}"
 export RUN_SYSTEM_TESTS="${RUN_SYSTEM_TESTS:-1}"
 export RUN_UNIT_TESTS="${RUN_UNIT_TESTS:-1}"
 export TARGET_ARCH="${TARGET_ARCH:-x86_64}"
@@ -49,6 +48,7 @@ unset PLATFORM
 
 # NOTE: The following variables are not set here because
 # they are specific to the CI implementation
+# PYTHON_EXECUTABLE
 # Z3_SRC_DIR
 # Z3_BUILD_DIR
 # Z3_SYSTEM_TEST_DIR

contrib/ci/scripts/travis_ci_osx_entry_point.sh

@@ -19,8 +19,9 @@ if [ ! -d "${TRAVIS_BUILD_DIR}" ]; then
   exit 1
 fi
 
-# These three variables are specific to the macOS TravisCI
+# These variables are specific to the macOS TravisCI
 # implementation and are not set in `ci_defaults.sh`.
+export PYTHON_EXECUTABLE="${PYTHON_EXECUTABLE:-$(which python)}"
 export Z3_SRC_DIR="${TRAVIS_BUILD_DIR}"
 export Z3_BUILD_DIR="${Z3_SRC_DIR}/build"
 export Z3_SYSTEM_TEST_DIR="${Z3_SRC_DIR}/z3_system_test"