[rust] set CARGO_HOME to $XDG_DATA_HOME/cargo

[cdmistman]

Why

this has been bothering me, and i just ran into a scenario where cloning a repo resulted in ~13k untracked files in git

we can't fix that with gitignore since a project's .cargo directory may also be checked into git by convention (primarily for project-local configuration, but there's other reasons to do so)

What changed

moved the CARGO_HOME env var from $REPL_HOME/.cargo to $XDG_DATA_HOME/cargo

Test plan

before:

1. clone https://github.com/tamasfe/taplo in replit
2. notice several thousand untracked files

after:

1. clone the same repo into a new repl
2. notice no untracked files
3. stat .local/share/cargo/registry has exit code 0

Rollout

Describe any procedures or requirements needed to roll this out safely (or check the box below)

• This is fully backward and forward compatible

[cdmistman]

this will have the unfortunate side-effect of rust repls having increased disk usage, but the solution is simple enough (rm -rdf $REPL_HOME/.cargo) that idt it's an issue