cli/build-rs: remove metadata command dependency in favor of env vars

When building this project with [Nix/Crane](ipetkov/crane#693), if the `jj-cli` dependency is specified in `Cargo.toml` as a git-based crate, `cargo vendor` splits this workspace up into sub-crate directories, which causes `cargo metadata` to fail when searching for relative deps in the workspace root.

This commit simply changes how the crate version is determined, using Cargo's built-in environment variable [`CARGO_PKG_VERSION`](https://doc.rust-lang.org/cargo/reference/environment-variables.html)

Cargo.toml

@@ -24,7 +24,6 @@ async-trait = "0.1.82"
 backoff = "0.4.0"
 blake2 = "0.10.6"
 bstr = "1.10.0"
-cargo_metadata = "0.17.0"
 clap = { version = "4.5.16", features = [
     "derive",
     "deprecated",

cli/Cargo.toml

@@ -47,9 +47,6 @@ required-features = ["test-fakes"]
 [[test]]
 name = "runner"
 
-[build-dependencies]
-cargo_metadata = { workspace = true }
-
 [dependencies]
 bstr = { workspace = true }
 chrono = { workspace = true }

cli/build.rs

@@ -16,20 +16,11 @@ use std::path::Path;
 use std::process::Command;
 use std::str;
 
-use cargo_metadata::MetadataCommand;
-
 const GIT_HEAD_PATH: &str = "../.git/HEAD";
 const JJ_OP_HEADS_PATH: &str = "../.jj/repo/op_heads/heads";
 
 fn main() -> std::io::Result<()> {
-    let path = std::env::var("CARGO_MANIFEST_DIR").unwrap();
-    let meta = MetadataCommand::new()
-        .manifest_path("./Cargo.toml")
-        .current_dir(&path)
-        .exec()
-        .unwrap();
-    let root = meta.root_package().unwrap();
-    let version = &root.version;
+    let version = std::env::var("CARGO_PKG_VERSION").unwrap();
 
     if Path::new(GIT_HEAD_PATH).exists() {
         // In colocated repo, .git/HEAD should reflect the working-copy parent.