Pin nightly compiler used in CI for uclibc

Workaround for rust-lang/rust#95866

.cirrus.yml

@@ -286,14 +286,29 @@ task:
     - name: OpenBSD x86_64
       env:
         TARGET: x86_64-unknown-openbsd
-    - name: Linux armv7 uclibceabihf
-      env:
-        TARGET: armv7-unknown-linux-uclibceabihf
   setup_script:
     - rustup component add rust-src
   << : *BUILD
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
+# uclibc needs its own task for now, due to Rust bug 95866
+# https://github.com/rust-lang/rust/issues/95866
+task:
+  name: Linux armv7 uclibceabihf
+  container:
+    image: rustlang/rust:nightly
+  env:
+    BUILD: check
+    ZFLAGS: -Zbuild-std
+    TARGET: armv7-unknown-linux-uclibceabihf
+    TOOLCHAIN: nightly-2022-04-01
+  setup_script:
+    - rustup toolchain install $TOOLCHAIN --profile minimal
+    - rustup component add --toolchain $TOOLCHAIN rust-src
+    - rustup component add --toolchain $TOOLCHAIN clippy
+  << : *BUILD
+  before_cache_script: rm -rf $CARGO_HOME/registry/index
+
 # Test that we can build with the lowest version of all dependencies.
 # "cargo test" doesn't work because some of our dev-dependencies, like
 # rand, can't build with their own minimal dependencies.