Use cpython3 host prebuilts on most branches

But continue building them on the aosp-build-tools branch, as that's
where the prebuilts come from.

This will allow us to link against openssl for host python, even though
openssl is only available on aosp-build-tools.

Bug: 173151817
Test: ensure prebuilts are used on master, and sources on build-tools
Test: treehugger
Change-Id: I568e84fb483a0cd2f949f65cd5392d252ee8d493
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index dfbe304..027dd30 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -47,7 +47,12 @@
     cat > ${SOONG_OUT}/soong.variables << EOF
 {
     "Allow_missing_dependencies": true,
-    "HostArch":"x86_64"
+    "HostArch":"x86_64",
+    "VendorVars": {
+        "cpython3": {
+            "force_build_host": "true"
+        }
+    }
 }
 EOF
     SOONG_BINARIES=(