fix: set variable CARGO_TARGET_DIR to build contract in contract project

[AlexD10S]

Closes #196

Fixes the bug reported above setting the env variable CARGO_TARGET_DIR before building the contract, as cargo-contract enforce the take the root_package to place the artifacts when building the contract. See here and here.

Added a unit test that ensures this variable is set.

[al3mart]

Looks great! Thanks 💖

[brunopgalvao]

Not sure I follow what @bolajahmad is saying. CARGO_TARGET_DIR should be used. In general, CARGO_TARGET_DIR is the location of where to place all generated artifacts, relative to the current working directory. See here.

IIU this PR correctly, the reason we want to set the CARGO_TARGET_DIR is because if the default is not specified then the directory named target located at the root of the workspace will be used. As stated here.

[AlexD10S]

Not sure I follow what @bolajahmad is saying. CARGO_TARGET_DIR should be used. In general, CARGO_TARGET_DIR is the location of where to place all generated artifacts, relative to the current working directory. See here.

IIU this PR correctly, the reason we want to set the CARGO_TARGET_DIR is because if the default is not specified then the directory named target located at the root of the workspace will be used. As stated here.

In the issue there is a discussion on how to fix it. But in this PR, you understood it correctly the fix is set the CARGO_TARGET_DIR variable to don't use the default one when generating the contract.
What is your concern? I'm open to considering other approaches.

[AlexD10S]

Closing it for now. Reason: #196 (comment)