Feature request: add possibility to only cache the ~/.cargo dir

[ttonelli-via]

My use case is I have a template repo that uses cargo-generate to scaffold out a project. The test workflow for this project installs cargo-generate and uses it on itself to create an instance of the template and tests against the output. The project itself is not particularly cacheable between builds, but I would like to be able to cache the ~/.cargo directory so that installing cargo-generate and other cargo subcommands is cached between builds.

I have tried setting the workspaces key to "", assuming that this would tell the action not to try to cache any workspaces, but I get the following error output:

Error: The process '/home/runner/.cargo/bin/cargo' failed with exit code 101
    at ExecState._setResult (/home/runner/work/_actions/Swatinem/rust-cache/v2/dist/restore/index.js:7780:25)
    at ExecState.CheckComplete (/home/runner/work/_actions/Swatinem/rust-cache/v2/dist/restore/index.js:7763:18)
    at ChildProcess.<anonymous> (/home/runner/work/_actions/Swatinem/rust-cache/v2/dist/restore/index.js:7657:27)
    at ChildProcess.emit (node:events:514:28)
    at maybeClose (node:internal/child_process:1105:16)
    at ChildProcess._handle.onexit (node:internal/child_process:305:5) {
  commandFailed: {
    command: 'cargo metadata --all-features --format-version 1 --no-deps',
    stderr: 'error: failed to load manifest for workspace member `/home/runner/work/stark-risc0-template/stark-risc0-template/lib`\n' +
      '\n' +
      'Caused by:\n' +
      '  failed to parse manifest at `/home/runner/work/stark-risc0-template/stark-risc0-template/lib/Cargo.toml`\n' +
      '\n' +
      'Caused by:\n' +
      '  no targets specified in the manifest\n' +
      '  either src/lib.rs, src/main.rs, a [lib] section, or [[bin]] section must be present\n'
  }
}

If this is just user error, that would be great to know. However, assuming this is feature is not available currently, would you be open to a PR that adds this feature?