CI: Pin VTest to a known good commit

As of January, 11th the macOS builds fail due to regression introduced in
VTest. This patch pins VTest to the newest good commit.

This patch should be reverted once VTest's 'master' is stable again.

see vtest/VTest#26

.github/workflows/vtest.yml

@@ -60,7 +60,7 @@ jobs:
         brew install lua
     - name: Install VTest
       run: |
-        curl -fsSL https://github.com/vtest/VTest/archive/master.tar.gz -o VTest.tar.gz
+        curl -fsSL https://github.com/vtest/VTest/archive/02a9bc16d78e70d1d415b40d5349b9f3e56df05b.tar.gz -o VTest.tar.gz
         mkdir VTest
         tar xvf VTest.tar.gz -C VTest --strip-components=1
         make -C VTest -j$(nproc) FLAGS="-O2 -s -Wall"