flatpak: Fix bubblewrap paths for icon-validator

Otherwise, `flatpak-validate-icon --sandbox` gives error:
  bwrap: Can't find source path /etc/ld.so.cache: No such file or directory

pkgs/development/libraries/flatpak/bubblewrap-paths.patch

@@ -0,0 +1,15 @@
+diff --git a/icon-validator/validate-icon.c b/icon-validator/validate-icon.c
+index 6e23d9f2..8c621ec4 100644
+--- a/icon-validator/validate-icon.c
++++ b/icon-validator/validate-icon.c
+@@ -149,8 +149,8 @@ rerun_in_sandbox (const char *arg_width,
+             "--unshare-ipc",
+             "--unshare-net",
+             "--unshare-pid",
+-            "--ro-bind", "/usr", "/usr",
+-            "--ro-bind", "/etc/ld.so.cache", "/etc/ld.so.cache",
++            "--ro-bind", "@storeDir@", "@storeDir@",
++            "--ro-bind", "/run/current-system", "/run/current-system",
+             "--ro-bind", validate_icon, validate_icon,
+             NULL);
+ 

pkgs/development/libraries/flatpak/default.nix

@@ -25,6 +25,10 @@ stdenv.mkDerivation rec {
       src = ./fix-paths.patch;
       p11 = p11-kit;
     })
+    (substituteAll {
+      src = ./bubblewrap-paths.patch;
+      inherit (builtins) storeDir;
+    })
     # patch taken from gtk_doc
     ./respect-xml-catalog-files-var.patch
     ./use-flatpak-from-path.patch