CI: Set DEBUG=-DDEBUG_STRICT=1 in GitHub Actions

This was missing when migrating from Travis.
diff --git a/.github/workflows/vtest.yml b/.github/workflows/vtest.yml
index 4d6fdfb..6d42db8 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -85,6 +85,7 @@
           ERR=1 \
           TARGET=${{ matrix.TARGET }} \
           CC=${{ matrix.CC }} \
+          DEBUG=-DDEBUG_STRICT=1 \
           ${{ join(matrix.FLAGS, ' ') }} \
           ADDLIB="-Wl,-rpath,/usr/local/lib/ -Wl,-rpath,$HOME/opt/lib/"
         sudo make install