feat(nix-reuse): Add installPath option

This allows installing the REUSE.toml file in a distinct directory from
the git toplevel

modules/reuse.nix

@@ -64,6 +64,14 @@ in
       readOnly = true;
     };
 
+    installPath = mkOption {
+      type = nullOr str;
+      default = null;
+      description = ''
+        Where to place the REUSE.toml generated file, if `null`, the file will be placed in the git toplevel directory.
+      '';
+    };
+
     defaultLicense = mkOption {
       type = nullOr str;
       default = null;
@@ -188,8 +196,9 @@ in
       }
     ];
 
-    installationScript =
-      # bash
+    installationScript = builtins.concatStringsSep "\n" [
+      (
+        if cfg.installPath == null then
           ''
             if ! type -t git >/dev/null; then
               # This happens in pure shells, including lorri
@@ -205,8 +214,15 @@ in
                 cp --no-preserve=mode,ownership ${result} "$GIT_WC/REUSE.toml" && echo "nix-reuse: Updated REUSE.toml"
               fi
             fi
-
-        ${optionalString config.downloadLicenses "${getExe pkgs.reuse} download --all"}
-      '';
+          ''
+        else
+          ''
+            if [ ! -f "${cfg.installPath}/REUSE.toml" ] || ! ${getExe' pkgs.diffutils "cmp"} -s "${cfg.installPath}/REUSE.toml" ${result} ; then
+              cp --no-preserve=mode,ownership ${result} "${cfg.installPath}/REUSE.toml" && echo "nix-reuse: Updated REUSE.toml"
+            fi
+          ''
+      )
+      (optionalString config.downloadLicenses "${getExe pkgs.reuse} download --all")
+    ];
   };
 }