Try to make the Clang containers work.

Change-Id: I6aa0129ba2f220b13d2502370f631ed25059caf9
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57801
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2108ce0..8a892b2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -22,10 +22,12 @@
       matrix:
         tag: [4.x, 5.x, 6.x, 7.x, 8.x, 9.x, 10.x, 11.x]
     env:
-      CC: clang
-      CXX: clang++
+      CC: /compiler/bin/clang
+      CXX: /compiler/bin/clang++
     steps:
     - uses: actions/checkout@v2
+    - run: apt update && apt install make
+      shell: bash
     - run: make && make test
       shell: bash
   build-gcc: