Set CARGO_HOME envvar in Makevars

[yutannihilation]

This is mandatory if you want to release this package on CRAN eventually. If not, you don't need to care.

https://github.com/r-rust/faq#how-to-avoid-writing-in-home

[sorhawell]

When moving cargo_home inside the package `R CMD check-warning like below may pop up depending on what dependencies.

I think it can be solved with some extra spring cleaning after cargo build, but before linking and R build finalizes.

I have been tinkering with builds and makevars for non stop 3 days.
To avoid madness, I will wait a week or so finalizing this ^^

* checking package subdirectories ... WARNING
Warning: Found the following directories with names of version control directories:
  ./src/.cargo/git/checkouts/arrow2-945af624853845da/baa2618/.git
  ./src/.cargo/git/checkouts/arrow2-945af624853845da/baa2618/testing/arrow-testing/.git
  ./src/.cargo/git/checkouts/arrow2-945af624853845da/baa2618/testing/parquet-testing/.git
  ./src/.cargo/git/checkouts/extendr-569c02e1a164cdf5/f511254/.git
  ./src/.cargo/git/checkouts/jsonpath-3ee2ec25574f7416/24eaf0b/.git
  ./src/.cargo/git/checkouts/polars-b0d90607192fd414/a7d38dd/.git
  ./src/.cargo/registry/index/github.com-1ecc6299db9ec823/.git

[sorhawell]

Reminder to myself
@eitsupi noticed I should use CURDIR instead of PWD in makefiles
#30 (comment)

[eitsupi]

Reopened by #340