4580: Fix invoking cargo without consulting CARGO env var or standard installation paths r=matklad a=Veetaha
Followup for #4329
The pr essentially fixes [this bug](https://youtu.be/EzQ7YIIo1rY?t=2189)
cc @lefticus
Co-authored-by: veetaha <veetaha2@gmail.com>
This is useful when an extension (e.g. Nix Environment Selector) or launch
configuration sets one or more environment variables.
When `env` is not explicitly specified in the options passed to
`child_process.spawn()` or `vscode.ShellExecution()`, then `process.env` gets
applied automatically. But when an explicit `env` is set, it should inherit from
`process.env` rather than replace it completely.
As per matklad, we now pass the responsibility for finding the binary to the frontend.
Also, added caching for finding the binary path to reduce
the amount of filesystem interactions.
