Set $PY to disambiguate which Py2.7 version to use

Interpreter constraints don't work, as previously noted.

.travis.yml

@@ -462,6 +462,8 @@ py27_osx_build_wheels_ucs4: &py27_osx_build_wheels_ucs4
     - PATH="/usr/local/opt/openssl/bin:$PATH" LDFLAGS="-L/usr/local/opt/openssl/lib" CPPFLAGS="-I/usr/local/opt/openssl/include" PYENV_ROOT="${HOME}/.pyenv" PATH="${PYENV_ROOT}/shims:${PATH}"
 
     - PYTHON_CONFIGURE_OPTS=--enable-unicode=ucs4
+    # We set $PY to ensure the UCS4 interpreter is used when bootstrapping the PEX.
+    - PY=${PYENV_ROOT}/shims/python2.7
   before_install:
     - curl -L https://github.com/stedolan/jq/releases/download/jq-1.5/jq-osx-amd64 -o /usr/local/bin/jq
     - chmod 755 /usr/local/bin/jq

build-support/docker/travis_ci_py27_ucs2/Dockerfile

@@ -9,7 +9,9 @@ FROM pantsbuild/centos6:latest
 
 # Note we use 2.7.15, rather than 2.7.13, as the centos6 image already comes with 2.7.13
 # installed, which uses UCS4 instead of UCS2. This allows us to disambiguate which Python 2
-# interpreter to use when `ci.sh` sets the interpreter constraints for Pants and PEX.
+# interpreter to use when `ci.sh` sets the interpreter constraints for Pants and PEX. We
+# set $PY to the exact Python 2.7 version we want to ensure the PEX is bootstrapped
+# with UCS 2.
 ARG PYTHON_2_VERSION=2.7.15
 RUN yum install sqlite-devel -y
 ENV PYENV_ROOT /pyenv-docker-build
@@ -19,7 +21,7 @@ ENV PYTHON_CONFIGURE_OPTS --enable-unicode=ucs2
 RUN /usr/bin/scl enable devtoolset-7 -- bash -c '\
     ${PYENV_ROOT}/bin/pyenv install ${PYTHON_2_VERSION} \
     && ${PYENV_ROOT}/bin/pyenv global ${PYTHON_2_VERSION}'
-ENV PATH "${PYENV_ROOT}/shims:${PATH}"
+ENV PY "${PYENV_ROOT}/shims/python2.7"
 
 # Setup mount points for the travis ci user & workdir.
 VOLUME /travis/home

build-support/travis/travis.yml.mustache

@@ -440,6 +440,8 @@ py27_osx_build_wheels_ucs4: &py27_osx_build_wheels_ucs4
     - CACHE_NAME=osxwheelsbuild.ucs4
     - {{>env_osx_with_pyenv}}
     - PYTHON_CONFIGURE_OPTS=--enable-unicode=ucs4
+    # We set $PY to ensure the UCS4 interpreter is used when bootstrapping the PEX.
+    - PY=${PYENV_ROOT}/shims/python2.7
   before_install:
     {{>before_install_osx}}
     - git clone https://github.com/pyenv/pyenv ${PYENV_ROOT}