fix: support relative TARGET_DIR in near-test-contracts

[matklad]

near-test-contracts builds some wasm contracts for use in testing. It
does so by recursively invoking cargo from build.rs. Before this
commit, we tried to re-use parent's CARGO_TARGET_DIR to figure out
where we should put the data. That was rather hacky, as cargo doesn't
expose that information to the build scripts in a reliable way:

rust-lang/cargo#9661 (comment)

Naturally, our hacks broken when when the CARGO_TARGET_DIR was set to
a relative path, because build.rs doesn't know where workspace root
lives.

The fix is to use OUT_DIR rather than CARGO_TARGET_DIR, which I
think is the supported way to this in the first place. Eg, the cc
crate uses OUT_DIR to store intermediate .o files, which I think
matches our use case pretty closely.

[Longarithm]

It seems to be conflicting with #6171

[matklad]

Yeah, I think this PR is still an improvement over that:

• overall, the approach is simpler and should be more robust: OUT_DIR is something supported by Cargo, while fetching TARGET_DIR is hacky
• this way, we'll actually honor the TARGET_DIR set from the outside (after Fix near-test-contracts build script to work if CARGO_TARGET_DIR is set. #6171, the code would actually be build into a local directory in src)

[matklad]

@nikurt could you merge this when the time is convenient? I thing this PR is a good idea, but, if #6171 works ok, and we are in the process of releasing something (which I don't know), I don't want to add extra posibility of something's going wrong just now.