use correct CARGO_HOME in test_rust.sh

make check currently doesn't seem to work with --enable-rust without --enable-cargo-online-mode.

It looks like cargo expects CARGO_HOME to point to the .cargo directory itself, not the directory containing it. Builds seem to work anyway because we cd to a directory where cargo can find a .cargo directory, even though we set CARGO_HOME incorrectly.

This might be making Jenkins builds fail too:

https://jenkins.torproject.org/job/tor-ci-linux-master-rust/ARCHITECTURE=i386,SUITE=sid/lastBuild/console

20:04:05 FAIL: src/test/test_rust.sh
20:04:05 ===========================
20:04:05 
20:04:05 error: attempting to make an HTTP request, but --frozen was specified
20:04:05 error: attempting to make an HTTP request, but --frozen was specified

This incarnation of this bug seems to be in the fix for legacy/trac#26258 (moved), but similar problems seem to exist in releases as far back as 0.3.1.