[Python] Fix staged installation of Python bindings

The problem is that with a single slash as in \$ENV{DESTDIR}, the variable
gets expanded too early (i.e., when passing ${PIP_OPTIONS} in the call to
install(CODE...)), so when "make DESTDIR=$INSTALL_DIR install" is run, the
variable is not part of the recipe, as $ENV{DESTDIR} has been passed to
install(CODE..) and has been expanded into an empty value. By adding one
more level of quoting, the expansion of DESTDIR happens at the right place
and the staged installation works correctly.

Closes: xrootd#1768

bindings/python/CMakeLists.txt

@@ -34,7 +34,7 @@ configure_file(${SETUP_PY_IN} ${SETUP_PY})
 
 string(FIND "${PIP_OPTIONS}" "--prefix" PIP_OPTIONS_PREFIX_POSITION)
 if( "${PIP_OPTIONS_PREFIX_POSITION}" EQUAL "-1" )
-  string(APPEND PIP_OPTIONS " --prefix \$ENV{DESTDIR}/${CMAKE_INSTALL_PREFIX}")
+  string(APPEND PIP_OPTIONS " --prefix \\\$ENV{DESTDIR}/${CMAKE_INSTALL_PREFIX}")
 else()
   message(WARNING
         " The pip option --prefix has been set in '${PIP_OPTIONS}' which will change"