llvm -> llvm_7 for nixos-unstable rustc

derivations.nix

@@ -16,11 +16,11 @@ rec {
   heavycomps = pkgs.callPackage ./heavycomps.nix { inherit nmigen; };
 
   binutils-riscv = pkgs.callPackage ./compilers/binutils.nix { platform = "riscv32"; };
-  llvm = pkgs.llvm_7.overrideAttrs(oa: {
+  llvm_7 = pkgs.llvm_7.overrideAttrs(oa: {
     cmakeFlags = oa.cmakeFlags ++ ["-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=RISCV"];
   });
   rustc = (pkgs.rustc.overrideAttrs(oa: {
     patches = oa.patches ++ [ ./compilers/rustc-riscv32i.patch ];
-  })).override { inherit llvm; };
+  })).override { inherit llvm_7; };
   rust-riscv32i-crates = pkgs.callPackage ./compilers/rust-riscv32i-crates.nix { inherit rustc; };
 }