Pin the nightly compiler version used with DragonflyBSD

Newer rustc is broken on DragonflyBSD, probably by rust-lang/rust@e68887e

.cirrus.yml

@@ -256,6 +256,31 @@ task:
   << : *BUILD
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
+# DragonflyBSD temporarily needs a pinned nightly toolchain
+# rustc is broken on DragonflyBSD as of Dec-5, probably by
+# https://github.com/rust-lang/rust/commit/e68887e67cc6b7bb4ea5113a40eaa4c0831bda13
+task:
+  container:
+    image: rust:1.46
+  name: DragonFly BSD x86_64
+  env:
+    BUILD: check
+    ZFLAGS: -Zbuild-std
+    TARGET: x86_64-unknown-dragonfly
+    # Redox requires a nightly compiler.
+    # If stuff breaks, change nightly to the date at
+    # https://gitlab.redox-os.org/redox-os/redox/-/blob/master/rust-toolchain
+    TOOLCHAIN: nightly-2021-12-04
+    # Temporarily allow deprecation on DragonflyBSD until an alternative is
+    # available.
+    #https://github.com/rust-lang/libc/pull/2522
+    RUSTFLAGS: -D warnings -A deprecated
+  setup_script:
+    - rustup toolchain add $TOOLCHAIN --profile minimal
+    - rustup component add rust-src
+  << : *BUILD
+  before_cache_script: rm -rf $CARGO_HOME/registry/index
+
 # Rust Tier 3 targets can't use Rustup
 task:
   container:
@@ -265,13 +290,6 @@ task:
     TOOLCHAIN: nightly
     ZFLAGS: -Zbuild-std
   matrix:
-    - name: DragonFly BSD x86_64
-      env:
-        TARGET: x86_64-unknown-dragonfly
-        # Temporarily allow deprecation on DragonflyBSD until an alternative is
-        # available.
-        #https://github.com/rust-lang/libc/pull/2522
-        RUSTFLAGS: -D warnings -A deprecated
     - name: OpenBSD x86_64
       env:
         TARGET: x86_64-unknown-openbsd