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/manifest.scm

@@ -25,6 +25,13 @@
              (guix profiles)
              (guix utils))
 
+(define-syntax-rule (search-our-patches file-name ...)
+  "Return the list of absolute file names corresponding to each
+FILE-NAME found in ./patches relative to the current file."
+  (parameterize
+      ((%patch-path (list (string-append (dirname (current-filename)) "/patches"))))
+    (list (search-patch file-name) ...)))
+
 (define (make-ssp-fixed-gcc xgcc)
   "Given a XGCC package, return a modified package that uses the SSP function
 from glibc instead of from libssp.so. Our `symbol-check' script will complain if
@@ -150,6 +157,10 @@ chain for " target " development."))
       (home-page (package-home-page pthreads-xgcc))
       (license (package-license pthreads-xgcc)))))
 
+(define (make-nsis-with-sde-support base-nsis)
+  (package-with-extra-patches base-nsis
+    (search-our-patches "nsis-SConstruct-sde-support.patch")))
+
 
 (packages->manifest
  (append
@@ -189,7 +200,9 @@ chain for " target " development."))
   (let ((target (getenv "HOST")))
     (cond ((string-suffix? "-mingw32" target)
            ;; Windows
-           (list zip (make-mingw-pthreads-cross-toolchain "x86_64-w64-mingw32") nsis-x86_64))
+           (list zip
+                 (make-mingw-pthreads-cross-toolchain "x86_64-w64-mingw32")
+                 (make-nsis-with-sde-support nsis-x86_64)))
           ((string-contains target "riscv64-linux-")
            (list (make-bitcoin-cross-toolchain "riscv64-linux-gnu"
                                                #:base-gcc-for-libc gcc-7)))

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()
+