Merge #6855

6855: Change recommendation when source can't be loaded from sysroot r=matklad a=flodiebold

Since we just tried running `rustup component add`, it doesn't make sense to me to recommend trying that again. If we're reaching this case, it's probably more likely that rustc was installed via package manager, in which case the source should be installed the same way (e.g. if you install the rust-src package on Ubuntu it will install a symlink in the right place to make our sysroot
detection work, and IMO we should get other distributors to do the same if they aren't already).

Co-authored-by: Florian Diebold <flodiebold@gmail.com>