refactor(bazel): use configured Rust toolchain root

bazel/utils.bzl

@@ -1,8 +1,8 @@
-def require(repository_ctx, executable):
-    path = repository_ctx.which(executable)
-    if path == None:
-        fail("{} must be available in PATH.".format(executable))
-    return path
+def require_env(repository_ctx, variable):
+    value = repository_ctx.os.environ.get(variable)
+    if not value:
+        fail("{} must be set.".format(variable))
+    return value
 
 
 def execute(repository_ctx, arguments):