Fix contracts compilation with `CARGO_TARGET_DIR` set (#2927)

In case `CARGO_TARGET_DIR` is set, build artifacts were in the wrong
place and `build.rs` was failing. With
`CARGO_TARGET_DIR=/home/nazar-pc/.cache/cargo/target`:
```
error: failed to run custom build command for `pallet-contracts-fixtures v1.0.0 (/web/subspace/polkadot-sdk/substrate/frame/contracts/fixtures)`

Caused by:
  process didn't exit successfully: `/home/nazar-pc/.cache/cargo/target/debug/build/pallet-contracts-fixtures-35d534f7ac3009e0/build-script-build` (exit status: 1)
  --- stderr
  Error: Failed to read "/tmp/.tmpiYwXfv/target/wasm32-unknown-unknown/release/dummy.wasm"

  Caused by:
      Can't read from the file: Os { code: 2, kind: NotFound, message: "No such file or directory" }
```

The file was actually in
`/home/nazar-pc/.cache/cargo/target/wasm32-unknown-unknown/release/dummy.wasm`.