ASDF_CRATE_DEFAULT_PACKAGES_FILE not consistent with other asdf plugins

[mcarifio]

First, ty for this plugin.

Looking at some of the other language plugins (python, nodejs), they provide env variables similar to ASDF_CRATE_DEFAULT_PACKAGES_FILE:

• ASDF_PYTHON_DEFAULT_PACKAGES_FILE
• ASDF_NPM_DEFAULT_PACKAGES_FILE

In each case, the env variable overrides the entire pathname, including the basename. You just reset the location. It's your script and you define the environment variable as you please. But it's not consistent with other plugin usage.

You might also want to mention that after asdf install rust, you should asdf reshim rust and rustup default stable. Probably need to do a reshim after cargo install as well. The reshim stuff can be a little confusing.

[hendursaga]

There is also a concern that the default location is not XDG-compliant..