Auto merge of #41170 - Nashenas88:master, r=alexcrichton

Use the existing path when removing the prefix fails

This allows the use of out-of-tree paths to be specified. I found this while trying to build with a modified version of `rls-data`, which is currently pointing to a version on crates.io.

cc @alexcrichton

Also, it wasn't clear if I needed to add a test for this (or how). I didn't see any tests that took paths into consideration.

diff --cc src/bootstrap/step.rs
Simple merge