build: Enable frame pointers for Rust modules

It seems to be necessary to build the aarch64 flatpak in CI, and it
doesn't hurt anyway.

build-aux/org.gnome.Fractal.Devel.json

@@ -23,6 +23,7 @@
         "append-ld-library-path": "/usr/lib/sdk/llvm18/lib",
         "append-path": "/usr/lib/sdk/llvm18/bin:/usr/lib/sdk/rust-stable/bin",
         "env": {
+            "RUSTFLAGS": "-C force-frame-pointers=yes",
             "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 --cfg=ruma_identifiers_storage=\"Arc\"",
             "CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER": "clang",