Restore lost "is_absolute" (#2922)

`is_absolute` was lost for "=" case in #2911

Fixes  #2917
diff --git a/cargo/private/cargo_build_script.bzl b/cargo/private/cargo_build_script.bzl
index eb91b0a..2c2a64a 100644
--- a/cargo/private/cargo_build_script.bzl
+++ b/cargo/private/cargo_build_script.bzl
@@ -182,7 +182,7 @@
             fix_next_arg = False
         else:
             opt, path = split_flag(arg)
-            if opt and path:
+            if opt and path and not paths.is_absolute(path):
                 res.append("{}${{pwd}}/{}".format(opt, path))
             else:
                 fix_next_arg = (opt != None)