Ensure that MSRV tests do not regress

When testing with older versions of rustc, there's a CI failure mode
when our dependency, which used to be compatible with that Rust version,
publishes a new version which can't be compiled using that old Rust
anymore. That's pretty unfortunate, as that means that third parties can
break our CI without any changes to the code in this repo.

To protect against that, let's use Cargo.lock on CI, but only when
testing against older versions. For stable Rust, we continue to generate
Cargo.lock with freshest dependencies. Implementation wise, we save
known-good Cargo.lock as `Cargo.lock.msrv`, and just copy that in every
relevant CI job.

To get a working `Cargo.lock.msrv`, I run `cargo +nightly
generate-lockfile -Zminimal-versions` -- that is guaranteed to support
the oldest Rust version we can.

.cirrus.yml

@@ -43,6 +43,7 @@ task:
     - . $HOME/.cargo/env
     - rustup target add i686-unknown-freebsd
     - rustup component add --toolchain $TOOLCHAIN clippy
+    - cp Cargo.lock.msrv Cargo.lock
   << : *TEST
   i386_test_script:
     - . $HOME/.cargo/env
@@ -64,6 +65,7 @@ task:
     - sh rustup.sh -y --profile=minimal --default-toolchain $TOOLCHAIN
     - . $HOME/.cargo/env
     - rustup component add --toolchain $TOOLCHAIN clippy
+    - cp Cargo.lock.msrv Cargo.lock
   << : *TEST
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
@@ -116,18 +118,13 @@ task:
     - sh rustup.sh -y --profile=minimal --default-toolchain $TOOLCHAIN
     - . $HOME/.cargo/env
     - cargo install cross
+    - cp Cargo.lock.msrv Cargo.lock
   << : *TEST
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
 # Tasks for Linux native builds
 task:
   matrix:
-    - name: Rust Stable
-      container:
-        image: rust:latest
-      env:
-        TARGET: x86_64-unknown-linux-gnu
-        TOOLCHAIN:
     - name: Linux aarch64
       arm_container:
         image: rust:1.46
@@ -144,6 +141,20 @@ task:
         image: rust:1.46
       env:
         TARGET: x86_64-unknown-linux-musl
+  setup_script:
+    - rustup target add $TARGET
+    - rustup component add clippy
+    - cp Cargo.lock.msrv Cargo.lock
+  << : *TEST
+  before_cache_script: rm -rf $CARGO_HOME/registry/index
+
+task:
+  name: Rust Stable
+  container:
+    image: rust:latest
+  env:
+    TARGET: x86_64-unknown-linux-gnu
+    TOOLCHAIN:
   setup_script:
     - rustup target add $TARGET
     - rustup component add clippy
@@ -219,13 +230,26 @@ task:
     - name: NetBSD x86_64
       env:
         TARGET: x86_64-unknown-netbsd
-    - name: Redox x86_64
-      env:
-        TARGET: x86_64-unknown-redox
-        # 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-2021-06-15
+  setup_script:
+    - rustup target add $TARGET
+    - rustup toolchain install $TOOLCHAIN --profile minimal --target $TARGET
+    - rustup component add --toolchain $TOOLCHAIN clippy
+    - cp Cargo.lock.msrv Cargo.lock
+  << : *BUILD
+  before_cache_script: rm -rf $CARGO_HOME/registry/index
+
+task:
+  container:
+    image: rust:1.46
+  env:
+    BUILD: check
+  name: Redox x86_64
+  env:
+   TARGET: x86_64-unknown-redox
+   # 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-2021-06-15
   setup_script:
     - rustup target add $TARGET
     - rustup toolchain install $TOOLCHAIN --profile minimal --target $TARGET