Disable cargo-hack in CI

Serde accidentally raised its MSRV to 1.51.0 in a patch release.  They
don't intent to fix it. Nix uses Serde via cargo-hack in CI.  Disable it
so we can publish a final release at 1.46.0.

Author: asomers

.cirrus.yml

@@ -11,6 +11,9 @@ env:
   # The MSRV
   TOOLCHAIN: 1.46.0
   ZFLAGS:
+  # Temporarily disable cargo-hack until we raise the MSRV to 1.51.0 or later.
+  # See https://github.com/nix-rust/nix/pull/1792
+  NOHACK: 1
 
 # Tests that don't require executing the build binaries
 build: &BUILD
@@ -64,7 +67,7 @@ task:
     - cargo test --target i686-unknown-freebsd
   i386_feature_script:
     - . $HOME/.cargo/env
-    - cargo hack check --each-feature --target i686-unknown-freebsd
+    - if [ -z "$NOHACK" ]; then cargo hack check --each-feature --target i686-unknown-freebsd; fi
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
 # Test macOS x86_64 in a full VM