pigweed.json: Add "relative_pigweed_root"

Bug: b/300632028
Change-Id: Ie5b9cc8b74370d9e4aadc861dc902f6cafad0beb
Reviewed-on: https://pigweed-review.googlesource.com/c/open-dice/+/171248
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/pigweed.json b/pigweed.json
index 11071ff..949ffa8 100644
--- a/pigweed.json
+++ b/pigweed.json
@@ -19,6 +19,7 @@
     },
     "pw_env_setup": {
       "root_variable": "OPEN_DICE_ROOT",
+      "relative_pigweed_root": "third_party/pigweed/src",
       "cipd_package_files": [
         "third_party/pigweed/src/pw_env_setup/py/pw_env_setup/cipd_setup/default.json"
       ],