Set `primary_source` for idenfitier in add3prf.py

In METADATA (go/thirdparty/metadata) there is an identifier required for
each package. Our script currently contains 2 identifier and therefore
it's unclear where we actually downloaded the package from.

Therefore, we need to set a `primary_source` marker. To quote the docs:

> The identifier that is the source of truth for the package should be marked with primary_source: true.

Bug: 321065092
Test: N/A
Change-Id: I39cb4ad842f0930d964d1f2b814d0f60d23d00bb
diff --git a/scripts/add3prf.py b/scripts/add3prf.py
index ff25952..6f808ef 100755
--- a/scripts/add3prf.py
+++ b/scripts/add3prf.py
@@ -75,6 +75,7 @@
   identifier {{
     type: "Archive"
     value: "https://static.crates.io/crates/{name}/{name}-{version}.crate"
+    primary_source: true
   }}
   version: "{version}"
   {license_comment}license_type: {license_type}