Auto merge of #7768 - chrisduerr:install-workspaces-from-git, r=ehuss

Auto merge of #7768 - chrisduerr:install-workspaces-from-git, r=ehuss

Search for root manifest with ephemeral workspaces

Fixes #5495.

This seems like it's too simple to just work like this, but after trying a few different things, this was the only solution which worked reliably for me.

I've verified that no `/target` is present in the actual checkout location, the target directory used is actually the one created in `/tmp`.

I've also verified that both workspaces and "normal" packages still install through git and that a normal `cargo install --path` works too (though that doesn't use ephemeral workspaces anyways).