ci(nix): switch to stable rust in shell.nix

Switch to https://github.com/oxalica/rust-overlay for rustc binaries.

Drop x86_64-unknown-linux-gnu from target list as host is included by
default.

ci/shell.nix

@@ -3,15 +3,15 @@
  }:
 
 let
-  mozillaOverlay = import (builtins.fetchTarball {
-    url = "https://github.com/mozilla/nixpkgs-mozilla/archive/8c007b60731c07dd7a052cce508de3bb1ae849b4.tar.gz";
-    sha256 = "1zybp62zz0h077zm2zmqs2wcg3whg6jqaah9hcl1gv4x8af4zhs6";
+  rustOverlay = import (builtins.fetchTarball {
+    url = "https://github.com/oxalica/rust-overlay/archive/4d5d8e4288a8e0efd074e56a448e450b0f8df975.tar.gz";
+    sha256 = "1v3xlpna3q5klnhrn3p43bdh24ynaf31s2980r98ypvdpi2wi018";
   });
   # the last successful build of nixpkgs-unstable as of 2021-05-07
   nixpkgs = import (builtins.fetchTarball {
     url = "https://github.com/NixOS/nixpkgs/archive/e62feb3bf4a603e26755238303bda0c24651e155.tar.gz";
     sha256 = "1gkamm044jrksjrisr7h9grg8p2y6rk01x6391asrx988hm2rh9s";
-  }) { overlays = [ mozillaOverlay ]; };
+  }) { overlays = [ rustOverlay ]; };
   moneroTests = nixpkgs.fetchurl {
     url = "https://github.com/ph4r05/monero/releases/download/v0.17.1.9-tests/trezor_tests";
     sha256 = "410bc4ff2ff1edc65e17f15b549bd1bf8a3776cf67abdea86aed52cf4bce8d9d";
@@ -22,11 +22,10 @@ let
     ${nixpkgs.patchelf}/bin/patchelf --set-interpreter "$(cat $NIX_CC/nix-support/dynamic-linker)" "$out"
     chmod -w $out
   '';
-  rustNightly = (nixpkgs.rustChannelOf { date = "2021-03-29"; channel = "nightly"; }).rust.override {
+  rustStable = nixpkgs.rust-bin.stable."1.52.1".default.override {
     targets = [
-      "x86_64-unknown-linux-gnu" # emulator
-      "thumbv7em-none-eabihf"    # TT
-      "thumbv7em-none-eabi"      # T1
+      "thumbv7em-none-eabihf" # TT
+      "thumbv7m-none-eabi"    # T1
     ];
   };
 in
@@ -64,7 +63,7 @@ stdenv.mkDerivation ({
     poetry
     protobuf3_6
     rustfmt
-    rustNightly
+    rustStable
     wget
     zlib
   ] ++ lib.optionals (!stdenv.isDarwin) [