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)
- [ ] I have made corresponding changes to the documentation (if
applicable)
- [ ] I have added tests that prove my fix is effective or that my
feature works (if applicable)

You can remove the "Checklist" section once all have been checked. Thank
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).