Do not expand environment variables (#98)

Only expand `racer-rust-src-path` and `racer-cargo-home` variables, not
`RUST_SRC_PATH` and `RUST_SRC_PATH` environment variables. This make the
behavior of racer the same in Emacs and on the command line with these variables
set. It will also allow `RUST_SRC_PATH` to be set to an empty string, which
is required with racer installed via nixpkgs, where `RUST_SRC_PATH` should not be set.

https://github.com/NixOS/nixpkgs/blob/348f938c552dcfdc41e543dd20511b5d0e4e70f6/pkgs/development/tools/rust/racer/rust-src.patch

racer.el

@@ -189,17 +189,19 @@ racer or racer.el."
   "Call racer command COMMAND with args ARGS.
 Return stdout if COMMAND exits normally, otherwise show an
 error."
-  (let ((rust-src-path (or racer-rust-src-path (getenv "RUST_SRC_PATH")))
-        (cargo-home (or racer-cargo-home (getenv "CARGO_HOME"))))
+  (let ((rust-src-path (or (when racer-rust-src-path (expand-file-name racer-rust-src-path))
+                           (getenv "RUST_SRC_PATH")))
+        (cargo-home (or (when racer-cargo-home (expand-file-name racer-cargo-home))
+                        (getenv "CARGO_HOME"))))
     (when (null rust-src-path)
       (user-error "You need to set `racer-rust-src-path' or `RUST_SRC_PATH'"))
     (unless (file-exists-p rust-src-path)
       (user-error "No such directory: %s. Please set `racer-rust-src-path' or `RUST_SRC_PATH'"
                   rust-src-path))
     (let ((default-directory (or (racer--cargo-project-root) default-directory))
           (process-environment (append (list
-                                        (format "RUST_SRC_PATH=%s" (expand-file-name rust-src-path))
-                                        (format "CARGO_HOME=%s" (expand-file-name cargo-home)))
+                                        (format "RUST_SRC_PATH=%s" rust-src-path)
+                                        (format "CARGO_HOME=%s" cargo-home))
                                        process-environment)))
       (-let [(exit-code stdout _stderr)
              (racer--shell-command racer-cmd (cons command args))]