Revert "CI: Build VTest with clang"

The issue with VTest not building properly in gcc is fixed since commit
vtest/VTest@0730540c43a2a23436b43f46327d6bac644d816d. Revert the patch to keep
the CI configuration simple.

This reverts commit e61f53eb44a390f9a8c8c4f34077c365942e0729.
diff --git a/.github/workflows/vtest.yml b/.github/workflows/vtest.yml
index 1d62f98..cb52f27 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -63,7 +63,7 @@
         curl -fsSL https://github.com/vtest/VTest/archive/master.tar.gz -o VTest.tar.gz
         mkdir VTest
         tar xvf VTest.tar.gz -C VTest --strip-components=1
-        make -C VTest -j$(nproc) FLAGS="-O2 -s -Wall" CC=clang
+        make -C VTest -j$(nproc) FLAGS="-O2 -s -Wall"
         sudo install -m755 VTest/vtest /usr/local/bin/vtest
     - name: Install SSL ${{ matrix.ssl }}
       if: ${{ matrix.ssl && matrix.ssl != 'stock' }}