[fractal] Use (fast) mold linker for developer flatpak variant

[Julian Sparber <jsparber src gnome org>]

commit 872a5f77d42c44969b94dd39ad51c30b773c5bef
Author: Julian Hofer <julianhofer gnome org>
Date:   Sat Apr 23 18:14:16 2022 +0200

    Use (fast) mold linker for developer flatpak variant
    
    This reduces incremental build time on my machine from 19 to 9 sec

 build-aux/org.gnome.Fractal.Hack.json | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)
---
diff --git a/build-aux/org.gnome.Fractal.Hack.json b/build-aux/org.gnome.Fractal.Hack.json
index 02d1d28e0..13733ee21 100644
--- a/build-aux/org.gnome.Fractal.Hack.json
+++ b/build-aux/org.gnome.Fractal.Hack.json
@@ -22,6 +22,12 @@
     "build-options": {
         "append-ld-library-path": "/usr/lib/sdk/llvm12/lib",
         "append-path": "/usr/lib/sdk/llvm12/bin:/usr/lib/sdk/rust-stable/bin",
+        "env": {
+            "CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER": "clang",
+            "CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS": "-C 
link-arg=-fuse-ld=/usr/lib/sdk/rust-stable/bin/mold",
+            "CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER": "clang",
+            "CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_RUSTFLAGS": "-C 
link-arg=-fuse-ld=/usr/lib/sdk/rust-stable/bin/mold"
+        },
         "build-args": [
             "--share=network"
         ],
@@ -62,4 +68,4 @@
             ]
         }
     ]
-}
\ No newline at end of file
+}