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`.