Trac #23059: Fix Python 2 build on Cygwin

[https://git.sagemath.org/sage.git/commit/?h=2179ac895c8d8aa9611837a0f53
84199e526bd3a&id=dc46ea2dfa97be79d7ef8ddbe9130d4c717acc3d This commit]
broke the build of Python 2 on Cygwin by moving the definition of the
`PYTHON_VERSION` variable used for the Cygwin install as well as OSX.

I didn't notice before since I was just doing incremental builds.

URL: https://trac.sagemath.org/23059
Reported by: embray
Ticket author(s): Erik Bray
Reviewer(s): Travis Scrimshaw

build/pkgs/python2/spkg-install

@@ -118,13 +118,14 @@ rm -f "$SAGE_LOCAL/lib/python"
 
 build
 
+PYTHON_VERSION=$("$SAGE_LOCAL/bin/python2" -c 'import sys; print("%d.%d" % sys.version_info[:2])')
+
 # On OS X with XCode 4, the presence of
 # $SAGE_LOCAL/lib/python2.*/config/libpython2.x.a causes problems with
 # GiNaC -- see #11967.  It is easiest to test the version of OS X; we
 # delete this file if using OS X 10.6 or later (so `uname -r` returns
 # x.y.z with x >= 10).
 if [ "$UNAME" = "Darwin" ] && \
-    PYTHON_VERSION=$("$SAGE_LOCAL/bin/python2" -c 'import sys; print("%d.%d" % sys.version_info[:2])')
     [ `uname -r | cut '-d.' -f1` -gt 9 ]; then
     rm -f "$SAGE_LOCAL/lib/python$PYTHON_VERSION/config/libpython${PYTHON_VERSION}.a"
 elif [ "$UNAME" = "CYGWIN" ]; then