Use nightly build

I need it to enable some attributes that are otherwise not available,
this all is to avoid

error[E0658]: `if` is not allowed in a `const`
-->
/home/mjost/.cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.43/src/int/specialized_div_rem/mod.rs:84:12
   |
84 |       } else if cfg!(any(target_arch = "sparc", target_arch =
"sparc64")) {
   |  ____^
85 | |         // LZD or LZCNT on SPARC only exists for the VIS 3
extension and later.
86 | |         cfg!(target_feature = "vis3")
87 | |     } else if cfg!(any(target_arch = "riscv32", target_arch =
"riscv64")) {
...  |
92 | |         true
93 | |     }
   | |___^
|
= note: see issue #49146
<rust-lang/rust#49146> for more information
= help: add `#![feature(const_if_match)]` to the crate attributes to
enable

rust-xtensa.nix

@@ -28,17 +28,17 @@ let
 
         # noted by inspecting https://static.rust-lang.org/dist/2020-03-12
         # version = "1.42.0";
-        # version = "nightly";
-        version = "beta";
+        version = "nightly";
+        # version = "beta";
 
         platform = toRustTarget pkgs.stdenv.hostPlatform;
         versionType = "bootstrap";
 
         src = pkgs.fetchurl {
             url = "https://static.rust-lang.org/dist/${date}/rust-${version}-${platform}.tar.gz";
             # sha256 = "0llhg1xsyvww776d1wqaxaipm4f566hw1xyy778dhcwakjnhf7kx"; # 1.42.0
-            # sha256 = "0jhggcwr852c4cqb4qv9a9c6avnjrinjnyzgfi7sx7n1piyaad43"; # nightly
-            sha256 = "1cv402wp9dx6dqd9slc8wqsqkrb7kc66n0bkkmvgjx01n1jhv7n5"; # beta
+            sha256 = "0jhggcwr852c4cqb4qv9a9c6avnjrinjnyzgfi7sx7n1piyaad43"; # nightly
+            # sha256 = "1cv402wp9dx6dqd9slc8wqsqkrb7kc66n0bkkmvgjx01n1jhv7n5"; # beta
         };
     };
     bootstrapPlatform = pkgs.makeRustPlatform rustBinary;