HACK ci: See if Linux builds work on clang and gcc snapshots

Author: real-or-random

.cirrus.yml

@@ -89,9 +89,9 @@ task:
     - env: { ECMULTGENPRECISION: 8, ECMULTWINDOW: 4 }
   matrix:
     - env:
-        CC: gcc
+        CC: gcc-snapshot
     - env:
-        CC: clang
+        CC: clang-snapshot
   << : *MERGE_BASE
   test_script:
     - ./ci/cirrus.sh