Consider CARGO_TARGET_DIR env variable

[phil-opp]

The CARGO_TARGET_DIR env variable can be set to override the directory where cargo should place the compilation artifacts (i.e. the target folder).

Setting this env variable can be useful on CI to work around disk space limitations. For example, the GitHub actions Windows CI runners have more disk space available on C: than D:, so you can move the target folder to the C: drive to avoid disk space errors on larger projects.

Unfortunately, the rust-cache action does not consider the CARGO_TARGET_DIR env variable and continues to cache a local target directory, which is now empty.

[phil-opp]

The workaround is quite simple fortunately:

      - uses: Swatinem/rust-cache@v2
        with:
          cache-directories: ${{ env.CARGO_TARGET_DIR }}