gnu: Update diffoscope to 121.

* gnu/packages/package-management (diffoscope): Update to 121.
  [version] Only define version once.
  [arguments] Add writeable-test-data phase.

gnu/packages/package-management.scm

@@ -561,18 +561,19 @@ transactions from C or Python.")
     (license license:gpl2+)))
 
 (define-public diffoscope
+  (let ((version "121"))
     (package
       (name "diffoscope")
-    (version "120")
+      (version version)
       (source (origin
                 (method git-fetch)
                 (uri (git-reference
                       (url "https://salsa.debian.org/reproducible-builds/diffoscope.git")
-                    (commit "120")))
+                      (commit version)))
                 (file-name (git-file-name name version))
                 (sha256
                  (base32
-                "07z9yclvfkw4326739l2ywzzihax5vdijiaqqpfix9rz1rb923aa"))))
+                  "1bw7s8qs1vnr93vhifl6pj6h6w6r6nrpc5anzhh9wx2gcaipkb3m"))))
       (build-system python-build-system)
       (arguments
        `(#:phases (modify-phases %standard-phases
@@ -606,6 +607,13 @@ transactions from C or Python.")
                           (("\\['getfacl',")
                            (string-append "['" (which "getfacl") "',")))
                         #t))
+                    (add-before 'check 'writable-test-data
+                      (lambda _
+                        ;; tests/comparators/test_elf.py needs write access to
+                        ;; test data
+                        (make-file-writable
+                         "tests/data/ignore_readelf_errors_expected_diff")
+                        #t))
                     (add-before 'check 'delete-failing-test
                       (lambda _
                         ;; this requires /sbin to be on the path
@@ -629,7 +637,7 @@ transactions from C or Python.")
 different.  It recursively unpacks archives of many kinds and transforms
 various binary formats into more human readable forms to compare them.  It can
 compare two tarballs, ISO images, or PDFs just as easily.")
-    (license license:gpl3+)))
+      (license license:gpl3+))))
 
 (define-public trydiffoscope
  (package