firmware: prevent Nix from corrupting output ELF

firmware/default.nix

@@ -7,6 +7,8 @@ rustPlatform.buildRustPackage rec {
   src = ./.;
   cargoSha256 = "0kf2pnfylfmzm0qvi4jfci43qvrnj1v5f037sb9zpvfqyddhdffl";
 
+  phases = ["unpackPhase" "buildPhase" "installPhase"];
+
   buildPhase = ''
     export CARGO_HOME=$(mktemp -d cargo-home.XXX)
     export RUSTFLAGS="-L ${rust-riscv32i-crates}/lib/rustlib/riscv32i-unknown-none-elf/lib -C linker=${binutils-riscv32}/bin/riscv32-unknown-elf-ld -C link-arg=-Tlink.x"