Pin nightly compiler used in CI for uclibc

Workaround for rust-lang/rust#95866

Author: asomers

.cirrus.yml

@@ -289,8 +289,13 @@ task:
     - name: Linux armv7 uclibceabihf
       env:
         TARGET: armv7-unknown-linux-uclibceabihf
+        # Pin nightly compiler due to
+        # https://github.com/rust-lang/rust/issues/95866
+        TOOLCHAIN: nightly-2022-04-01
   setup_script:
     - rustup component add rust-src
+    # install the pinned compiler, if necessary
+    - rustup toolchain install $TOOLCHAIN --profile minimal
   << : *BUILD
   before_cache_script: rm -rf $CARGO_HOME/registry/index