Add Clang 12 to the build matrix.

Change-Id: Ib5c2318f95336f16dade15d38a09cd301b60a1ae
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58490
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index bfc4c97..9ac25ff 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,7 +21,7 @@
     strategy:
       fail-fast: false
       matrix:
-        tag: [9, 10, 11]
+        tag: [9, 10, 11, 12]
     env:
       CC: clang-${{ matrix.tag }}
       CXX: clang++-${{ matrix.tag }}