Merge #20937: guix: Make nsis reproducible by respecting SOURCE-DATE-EPOCH

1fca9811e1 lint: Skip whitespace lint for guix patches (Carl Dong)
a91c46c57d guix: Make nsis reproducible by respecting SOURCE-DATE-EPOCH (Carl Dong)

Pull request description:

  ```
  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
  ```

  Sidenote: also a good demonstration of how Guix allows us to flexibly patch our tools!

  Note to reviewers: if you want to compare hashes, please build after Jan 16th 2021 without my substitute server enabled!

ACKs for top commit:
  fanquake:
    ACK 1fca9811e1

Tree-SHA512: b800e0ce5f73827ad353739effb9167ec3a6bdb362c725ae20dd3f025ce78660f85c70ce1d75cd0896facf1e8fe38a9e058459ed13dec71ab3a2fe41e20eaa5d

test/lint/lint-whitespace.sh

@@ -33,7 +33,7 @@ if [ -z "${COMMIT_RANGE}" ]; then
 fi
 
 showdiff() {
-  if ! git diff -U0 "${COMMIT_RANGE}" -- "." ":(exclude)depends/patches/" ":(exclude)src/leveldb/" ":(exclude)src/crc32c/" ":(exclude)src/secp256k1/" ":(exclude)src/univalue/" ":(exclude)doc/release-notes/" ":(exclude)src/qt/locale/"; then
+  if ! git diff -U0 "${COMMIT_RANGE}" -- "." ":(exclude)depends/patches/" ":(exclude)contrib/guix/patches/" ":(exclude)src/leveldb/" ":(exclude)src/crc32c/" ":(exclude)src/secp256k1/" ":(exclude)src/univalue/" ":(exclude)doc/release-notes/" ":(exclude)src/qt/locale/"; then
     echo "Failed to get a diff"
     exit 1
   fi