RC5 Runtime Failure NixOS

## Specs

[x] Operating System: NixOS 26.05 \
[x] Desktop Environment: niriWM \
[x] Terminal Emulator: foot \
[x] Shell: fish 4.6.0 \
[x] Software Version: RC5

## Description
When attempting to build the rc5 candidate on nixos, the build errors out due to
an non-writeable `/tmp` directory.

One can work around this by setting `$TEMP` themselves:

```sh
mkdir rc_temp
TEMP=rc_temp nix run github:GraphiteEditor/Graphite
```

## Steps to reproduce
1. Use `nix run` to build the rc candidate:

```sh
nix run github:GraphiteEditor/Graphite
```

## Expected behavior

The project would build and run correctly according to the provided
instructions.

## Screenshots

## Logs

```sh
Available WGPU adapters:
[0] Vulkan IntegratedGpu (Name: AMD Radeon 780M Graphics (RADV PHOENIX), Driver: radv, Device: 5567)
[1] Vulkan Cpu (Name: llvmpipe (LLVM 21.1.8, 256 bits), Driver: llvmpipe, Device: 0)
[2] Gl Other (Name: AMD Radeon 780M Graphics (radeonsi, phoenix, ACO, DRM 3.64, 6.19.11), Driver: , Device: 0)

Using WGPU adapter: AdapterInfo { name: "AMD Radeon 780M Graphics (RADV PHOENIX)", vendor: 4098, device: 5567, device_type: IntegratedGpu, device_pci_bus_id: "0000:c1:00.0", driver: "radv", driver_info: "Mesa 26.0.4", backend: Vulkan, subgroup_min_size: 32, subgroup_max_size: 64, transient_saves_memory: false }

thread 'main' (990551) panicked at desktop/src/cef/context/builder.rs:79:43:
Failed to create temporary directory for CEF instance: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }
stack backtrace:
0: 0x5646fac222ea - ::fmt::h530ac34e196461be
1: 0x5646f92c401a - core::fmt::write::hfebbe83a528033c1
2: 0x5646fac22d36 - std::io::default_write_fmt::h0ba17d3fe335279a
3: 0x5646fac1f8a3 - std::panicking::default_hook::{{closure}}::h19be2464c5c0a285
4: 0x5646fac1f745 - std::panicking::default_hook::h613c80136145dcc7
5: 0x5646fac1fabb - std::panicking::panic_with_hook::hb09a36a2026ab877
6: 0x5646fac1f948 - std::panicking::panic_handler::{{closure}}::ha4299fe6dc67fbbc
7: 0x5646fac1f119 - std::sys::backtrace::__rust_end_short_backtrace::hafec91f98267cfd4
8: 0x5646fac1b2ed - __rustc[cba8e45071337d10]::rust_begin_unwind
9: 0x5646f8f877ac - core::panicking::panic_fmt::h41b75fcbafe65d21
10: 0x5646f8f874e2 - core::result::unwrap_failed::hec126d4a1961cb02
11: 0x5646f95c97e3 - graphite_desktop::cef::context::builder::CefContextBuilder::create::h6fc30a111a649d11
12: 0x5646f95db99e - graphite_desktop::start::h0137246f15ca12fc
13: 0x5646f9575d03 - std::sys::backtrace::__rust_begin_short_backtrace::ha164532b5092f231
14: 0x5646f9575cf9 - std::rt::lang_start::{{closure}}::hf9faa02f67bbe4c7
15: 0x5646fac1de27 - std::rt::lang_start_internal::h68c86ce285fb0520
16: 0x5646f9575d5c - main
17: 0x7fca1f42b285 - __libc_start_call_main
18: 0x7fca1f42b338 - __libc_start_main_alias_2
19: 0x5646f91aa505 - _start
20: 0x0 -
```

## Environment

/etc/nixos/configuration.nix excerpt

```nix
nix.settings = {
substituters = [
# "https://graphite.cachix.org"
"https://nix-community.cachix.org"
];
trusted-public-keys = [
# "graphite.cachix.org-1:B7Il1yMpkquN/dXM+5GRmz+4Xmu2aaCS1GcWNfFhsOo="
"nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
];

experimental-features = [
"nix-command"
"flakes"
];
allowed-users = [
"@wheel"
"fugue"
];
trusted-users = [
"root"
"fugue"
"@wheel"
];
};
```