cargo: make jj installable with cargo binstall

https://github.com/cargo-bins/cargo-binstall

Note that `jj` will only become installable once the next release
is published to crates.io. For this reason, I am planning to
wait until then before documenting the fact that `jj` can be
installed this way.

At that point, `cargo binstall jj-cli` should be sufficient.

Before then, it's possible to test that this will work by doing

```
cargo binstall jj-cli --force --strategies crate-meta-data --log-level debug  --dry-run --manifest-path cli/Cargo.toml
``` 

Without --dry-run, this should install the 0.9 release if run
on `cli/Cargo.toml` form this commit.

cli/Cargo.toml

@@ -84,3 +84,8 @@ packaging = []
 test-fakes = []
 vendored-openssl = ["git2/vendored-openssl", "jj-lib/vendored-openssl"]
 watchman = ["jj-lib/watchman"]
+
+[package.metadata.binstall]
+# The archive name is jj, not jj-cli. Also, `cargo binstall` gets
+# confused by the `v` before versions in archive name.
+pkg-url="{ repo }/releases/download/v{ version }/jj-v{ version }-{ target }.{ archive-format }"