gnu: json-modern-cxx: Remove input labels.

* gnu/packages/cpp.scm (json-modern-cxx)[native-inputs]: Remove labels.
[arguments]: Use SEARCH-INPUT-FILE.

gnu/packages/cpp.scm

@@ -528,7 +528,9 @@ as ordering relation.")
      '(#:configure-flags
        (list "-DJSON_MultipleHeaders=ON" ; For json_fwd.hpp.
              (string-append "-DJSON_TestDataDirectory="
-                            (assoc-ref %build-inputs "json_test_data")))
+                            (dirname
+                             (search-input-directory %build-inputs
+                                                     "json_nlohmann_tests"))))
        #:phases (modify-phases %standard-phases
                   (replace 'check
                     (lambda* (#:key tests? parallel-tests? #:allow-other-keys)
@@ -540,10 +542,8 @@ as ordering relation.")
                                            "1"))
                           (format #t "test suite not run~%")))))))
     (native-inputs
-     `(("amalgamate" ,amalgamate)
-       ("doctest" ,doctest)
-       ("json_test_data"
-        ,(let ((version "3.0.0"))
+     (list amalgamate doctest
+           (let ((version "3.0.0"))
              (origin
                (method git-fetch)
                (uri (git-reference
@@ -552,7 +552,7 @@ as ordering relation.")
                (file-name (git-file-name "json_test_data" version))
                (sha256
                 (base32
-               "0nzsjzlvk14dazwh7k2jb1dinb0pv9jbx5jsyn264wvva0y7daiv")))))))
+                 "0nzsjzlvk14dazwh7k2jb1dinb0pv9jbx5jsyn264wvva0y7daiv"))))))
     (inputs
      (list fifo-map))
     (synopsis "JSON parser and printer library for C++")