guix: Make nsis reproducible by respecting SOURCE-DATE-EPOCH

When building nsis, if VERSION is not specified, it defaults to
cvs_version which is non-deterministic as it includes the current date.

This patches nsis to default to SOURCE_DATE_EPOCH if it exists so that
nsis is reproducible.

Upstream change: https://github.com/kichik/nsis/pull/13

contrib/guix/patches/nsis-SConstruct-sde-support.patch

@@ -0,0 +1,15 @@
+diff --git a/SConstruct b/SConstruct
+index e8252c9..41786f2 100755
+--- a/SConstruct
++++ b/SConstruct
+@@ -95,8 +95,8 @@ default_doctype = 'html'
+ if defenv.WhereIs('hhc', os.environ['PATH']):
+ 	default_doctype = 'chm'
+ 
+-from time import strftime, gmtime
+-cvs_version = strftime('%d-%b-%Y.cvs', gmtime())
++import time
++cvs_version = time.strftime('%d-%b-%Y.cvs', time.gmtime(int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))))
+ 
+ opts = Variables()
+