Fix the CI build with older Rust versions

When testing with the MSRV (1.46.0) or the Debian Rust package (1.48.0),
we must lock predicates-core and predicates-tree at older versions.  Add
a Cargo.lock.msrv file and use it in CI.  But it will not be published
to crates.io.

Author: asomers

.cirrus.yml

@@ -22,6 +22,7 @@ rust_test_task:
   cargo_cache:
     folder: $CARGO_HOME/registry
   build_script:
+    - rustc --version | grep -q 1.46.0 && mv plugins/rust/Cargo.lock.msrv plugins/rust/Cargo.lock
     - cd plugins/rust; cargo build $CARGO_ARGS
   test_script:
     - cd plugins/rust; cargo test $CARGO_ARGS

plugins/rust/Cargo.lock.msrv

@@ -9,7 +9,7 @@ repository = "https://gitlab.com/nbdkit/nbdkit"
 categories = ["api-bindings", "filesystem"]
 keywords = ["network-block-device", "nbd"]
 homepage = "http://libguestfs.org/"
-exclude = ["nbdkit-rust-plugin.*", "Makefile*", "test.sh"]
+exclude = ["nbdkit-rust-plugin.*", "Makefile*", "test.sh", "Cargo.lock.msrv"]
 description = """
 Rust bindings to the NBDKit framework for creating Network Block Device servers
 """