wasm-builder: manually set CARGO_TARGET_DIR (#1951)

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# Description

*Please include a summary of the changes and the related issue. Please
also include relevant motivation and context,
including:*

- What does this PR do?

make 'substrate-wasm-builder' manually set 'CARGO_TARGET_DIR' to
'$project_dir/target' while building instead of unset
'CARGO_TARGET_DIR';

- Why are these changes needed?

If you using this in the `build.rs` with following content in your
`~/.cargo/config.toml':

    [build]
    target-dir = "target"

the build process will stuck because of dead lock -- two `cargo build`
on same target directory in the same time.
There is already an attempt to avoid such dead lock by unset the
`CARGO_TARGET_DIR`, but for users with config above in his build
enviroment (like me), this workaround won't work.

- How were these changes implemented and what do they affect?

Instead of unset 'CARGO_TARGET_DIR', we set 'CARGO_TARGET_DIR' to
'$project/target/', which is already assumed to be true by rest of the
code.

*Use [Github semantic

linking](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword)
to address any open issues this PR relates to or closes.*

Fixes # (issue number, *if applicable*)

Closes # (issue number, *if applicable*)

# Checklist

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"Description" section above
- [ ] My PR follows the [labeling requirements](CONTRIBUTING.md#Process)
of this project (at minimum one label for `T`
  required)
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- [ ] I have added tests that prove my fix is effective or that my
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you for your contribution!

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I have built my project with this fix, there's still some warnings with
`build.target-dir` set but the building process won't hang.
I haven't found related issue in this repo. But I did find one issue
[here](https://github.com/substrate-developer-hub/substrate-node-template/issues/116).

substrate/utils/wasm-builder/src/prerequisites.rs

@@ -143,9 +143,10 @@ fn check_wasm_toolchain_installed(
 	run_cmd.current_dir(&temp);
 	run_cmd.args(&["run", "--manifest-path", &manifest_path]);
 
-	// Unset the `CARGO_TARGET_DIR` to prevent a cargo deadlock
-	build_cmd.env_remove("CARGO_TARGET_DIR");
-	run_cmd.env_remove("CARGO_TARGET_DIR");
+	// manually set the `CARGO_TARGET_DIR` to prevent a cargo deadlock
+	let target_dir = temp.path().join("target").display().to_string();
+	build_cmd.env("CARGO_TARGET_DIR", &target_dir);
+	run_cmd.env("CARGO_TARGET_DIR", &target_dir);
 
 	// Make sure the host's flags aren't used here, e.g. if an alternative linker is specified
 	// in the RUSTFLAGS then the check we do here will break unless we clear these.

substrate/utils/wasm-builder/src/wasm_project.rs

@@ -670,10 +670,10 @@ fn build_project(
 		.args(&["rustc", "--target=wasm32-unknown-unknown"])
 		.arg(format!("--manifest-path={}", manifest_path.display()))
 		.env("RUSTFLAGS", rustflags)
-		// Unset the `CARGO_TARGET_DIR` to prevent a cargo deadlock (cargo locks a target dir
+		// Manually set the `CARGO_TARGET_DIR` to prevent a cargo deadlock (cargo locks a target dir
 		// exclusive). The runner project is created in `CARGO_TARGET_DIR` and executing it will
 		// create a sub target directory inside of `CARGO_TARGET_DIR`.
-		.env_remove("CARGO_TARGET_DIR")
+		.env("CARGO_TARGET_DIR", &project.join("target").display().to_string())
 		// As we are being called inside a build-script, this env variable is set. However, we set
 		// our own `RUSTFLAGS` and thus, we need to remove this. Otherwise cargo favors this
 		// env variable.