devshell: fix usage with non-nix rust-analyzer

rust-analyzer binaries that don't come from nix (such as the one
shipped with vscode's rust-analyzer extension by default) need the
`RUST_SRC_PATH` env var to be the path to rustc's sources, but this
wasn't set inside the devshell. this change explicitely adds it to
the environment. (trying to add pkgs.rustLibSrc and rustcSrc to
the shell's packages unfortunately does nothing, because they are
just pure source packages, nothing more.)

(the reason this probably didn't come up for other devs is because
1. a lot of lix devs use other editors that probably just use the
binary from the path, and the devshell installs pkgs.rust-analyzer,
which is patched to always have `RUST_SRC_PATH` set, and 2. because
pkgs.vscode-extensions.rust-lang.rust-analyzer is also patched to use
pkgs.rust-analyzer by default instead of the bundled one, so installing
the extension using nix instead of from the marketplace would also hide
this problem.)

Change-Id: Ia27daf2a88756311ac5485ea0af44aae242241a9

package.nix

@@ -833,7 +833,10 @@ stdenv.mkDerivation (finalAttrs: {
           inputsFrom = [ finalAttrs.finalPackage ];
 
           # For Meson to find Boost.
-          env = finalAttrs.env;
+          env = finalAttrs.env // {
+            # for non-nixpkgs rust-analyzer binaries, we need to epxlicitely set RUST_SRC_PATH
+            RUST_SRC_PATH = "${rustPlatform.rustLibSrc}";
+          };
 
           mesonFlags =
             # I guess this is necessary because mesonFlags to mkDerivation doesn't propagate in inputsFrom,
@@ -890,8 +893,6 @@ stdenv.mkDerivation (finalAttrs: {
               cargo
               rustc
               rustfmt
-              rustPlatform.rustLibSrc
-              rustPlatform.rustcSrc
             ]
             ++ lib.optionals stdenv.cc.isClang [
               # Required for clang-tidy checks.