Avoid choosing virtualenv when building PyPy from source

plugins/python-build/bin/pyenv-install

@@ -207,9 +207,13 @@ if [ -z "${PYENV_BOOTSTRAP_VERSION}" ]; then
     else
       for version in $(pyenv-versions --bare | sort -r); do
         if [[ "${version}" == "2.7"* ]]; then
+          if [ -f "$(pyenv-prefix "${version}")/bin/activate" ]; then
+            : # avoid choosing virtualenv since it may not have some necessary modules (e.g. cffi, pycparser)
+          else
             PYENV_BOOTSTRAP_VERSION="$version"
             break
           fi
+        fi
       done
     fi
     if [ -n "$PYENV_BOOTSTRAP_VERSION" ]; then