CI [drop-before-merge]: temporarily disable the existing test matrix

Set `if: false` on the legacy `test` job so we don't burn CI minutes on
the full OS x Python matrix while iterating on the new cross-repo PR
workflow. The `discover-cross-repo-prs`, `build-archive-image`, and
`test-cross-repo` jobs remain active.

This commit is intended to be dropped before this PR (or its successor)
lands on master.

Co-Authored-By: Claude Code 2.1.114 / Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yarikoptic

.github/workflows/run-tests.yml

@@ -237,6 +237,10 @@ jobs:
           fail_ci_if_error: false
 
   test:
+    # TODO(remove-before-merge): temporarily disabled to expedite iteration
+    # on the cross-repo PR matrix feature. Drop this `if: false` (and the
+    # whole commit that introduced it) before merging.
+    if: false
     runs-on: ${{ matrix.os }}
     env:
       NO_ET: 1