chromium: move export in preConfigure to env.

Better late than never. This was intended to happen a long time ago.

pkgs/applications/networking/browsers/chromium/common.nix

@@ -812,15 +812,7 @@ let
       // (extraAttrs.gnFlags or { })
     );
 
-    # TODO: Migrate this to env.RUSTC_BOOTSTRAP next mass-rebuild.
-    # Chromium expects nightly/bleeding edge rustc features to be available.
-    # Our rustc in nixpkgs follows stable, but since bootstrapping rustc requires
-    # nightly features too, we can (ab-)use RUSTC_BOOTSTRAP here as well to
-    # enable those features in our stable builds.
-    preConfigure = ''
-      export RUSTC_BOOTSTRAP=1
-    ''
-    + lib.optionalString (!isElectron) ''
+    preConfigure = lib.optionalString (!isElectron) ''
       (
         cd third_party/node
         grep patch update_npm_deps | sh
@@ -841,6 +833,11 @@ let
       runHook postConfigure
     '';
 
+    # Chromium expects nightly/bleeding edge rustc features to be available.
+    # Our rustc in nixpkgs follows stable, but since bootstrapping rustc requires
+    # nightly features too, we can (ab-)use RUSTC_BOOTSTRAP here as well to
+    # enable those features in our stable builds.
+    env.RUSTC_BOOTSTRAP = 1;
     # Mute some warnings that are enabled by default. This is useful because
     # our Clang is always older than Chromium's and the build logs have a size
     # of approx. 25 MB without this option (and this saves e.g. 66 %).