Set CARGO_HOME in Makevars

[yutannihilation]

I got this review comment from CRAN:

Checking this creates ~/.cargo sized 82MB, in violation of the CRAN Policy. Please fix as necessary and resubmit.

It seems we need to set CARGO_HOME to somewhere inside package directory and delete the cache after installation.

c.f. https://github.com/r-rust/gifski/blob/6b86cc6b60abbc2294db821f27cae37413df70c2/src/Makevars#L9