fix: Set SYSROOT environment variable to the directory of the rust_lib files (#786)

This change fixes some issues that the Kythe team is having building our Rust extractor inside Bazel

Author: wcalandro

docs/BUILD.bazel

@@ -11,6 +11,7 @@ bzl_library(
         "@build_bazel_rules_nodejs//internal/providers:bzl",
     ],
     deps = [
+        "@bazel_skylib//lib:paths",
         "@rules_proto//proto:rules",
     ],
 )

rust/private/rustc.bzl

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 # buildifier: disable=module-docstring
+load("@bazel_skylib//lib:paths.bzl", "paths")
 load(
     "@bazel_tools//tools/build_defs/cc:action_names.bzl",
     "CPP_LINK_EXECUTABLE_ACTION_NAME",
@@ -487,9 +488,8 @@ def construct_arguments(
         data_paths,
     ))
 
-    # This empty value satisfies Clippy, which otherwise complains about the
-    # sysroot being undefined.
-    env["SYSROOT"] = ""
+    # Set the SYSROOT to the directory of the rust_lib files passed to the toolchain
+    env["SYSROOT"] = paths.dirname(toolchain.rust_lib.files.to_list()[0].short_path)
 
     return args, env