Update Redox compiler.

The latest libc uses the native_link_modifiers feature, which isn't
known by the old compiler used in the Redox builds.  Update Redox's
compiler to that used by the Redox project itself.

.cirrus.yml

@@ -223,9 +223,9 @@ task:
   env:
     TARGET: x86_64-unknown-redox
     # Redox requires a nightly compiler.
-    # If stuff breaks, change nightly to the date in the toolchain_*
-    # directory at https://static.redox-os.org
-    TOOLCHAIN: nightly-2020-08-04
+    # If stuff breaks, change nightly to the date at
+    # https://gitlab.redox-os.org/redox-os/redox/-/blob/master/rust-toolchain
+    TOOLCHAIN: nightly-2021-06-15
   container:
     image: rustlang/rust:nightly
   setup_script: