Merge #7643 #7663

7643: Automatically detect the rustc-src directory (fixes #3517) r=matklad a=bnjbvr

If the configured rustcSource was not set, then try to automatically
detect a source for the sysroot rustc directory.

I wasn't sure how to do it in the case of the project.json file, though.

7663: Tolerate spaces in nix binary patching r=matklad a=CertainLach

If path to original file contains space (I.e on code insiders, where
default data directory is ~/Code - Insiders/), then there is syntax
error evaluating src arg.

Instead pass path as str, and coerce to path back in nix expression

Co-authored-by: Benjamin Bouvier <[email protected]>
Co-authored-by: Yaroslav Bolyukin <[email protected]>