rustPlatform.importCargoLock: fix workspace_root detection

Prior to this change, the `importCargoLock` git dependency builder
assumed that the workspace root that needed to be passed to
`replace-workspace-values` will always be the root directory of the git
repository.

This is not always the case as independent workspace roots may be used
from subdirectories, and packages be referenced via paths. An example of
this is [1], where the `iced` subdirectory is its own independent
workspace, and workspace values for packages within it should be pulled
from the `iced` subdirectory as the workspace root, not from the
top-level of the fetched repository.

[1]: https://github.com/pop-os/libcosmic/blob/b8f1a366dd030b90ed72e50f521e3da1d6a676ce/Cargo.toml

pkgs/build-support/rust/import-cargo-lock.nix

@@ -193,7 +193,7 @@ let
 
         if grep -q workspace "$out/Cargo.toml"; then
           chmod u+w "$out/Cargo.toml"
-          ${replaceWorkspaceValues} "$out/Cargo.toml" "${tree}/Cargo.toml"
+          ${replaceWorkspaceValues} "$out/Cargo.toml" "$(${cargo}/bin/cargo metadata --format-version 1 --no-deps --manifest-path $crateCargoTOML | ${jq}/bin/jq -r .workspace_root)/Cargo.toml"
         fi
 
         # Cargo is happy with empty metadata.