Auto merge of #18229 - mrkajetanp:rustfmt-path, r=Veykril

fix: Join rustfmt overrideCommand with project root

When providing a custom rustfmt command, join it with the project root instead of the workspace root. This fixes rust-analyzer getting the wrong invocation path in projects containing subprojects.

This makes the behaviour consistent with how a custom path provided in rust-analyzer.procMacro.server behaves already.

Resolves issue #18222

Author: bors