Change recommendation when source can't be loaded from sysroot

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).

crates/project_model/src/sysroot.rs

@@ -143,7 +143,7 @@ fn discover_sysroot_src_dir(current_dir: &AbsPath) -> Result<AbsPathBuf> {
 can't load standard library from sysroot
 {}
 (discovered via `rustc --print sysroot`)
-try running `rustup component add rust-src` or set `RUST_SRC_PATH`",
+try installing the Rust source the same way you installed rustc",
                 sysroot_path.display(),
             )
         })