Provide a way to compile with c++20

This allows us to verify if we're ready for the next c++
version.

Signed-off-by: Henner Zeller <hzeller@google.com>

.github/bin/build-and-test.sh

diff --git a/.github/bin/build-and-test.sh b/.github/bin/build-and-test.sh
index aca1978..7ab9ec1 100755
--- a/.github/bin/build-and-test.sh
+++ b/.github/bin/build-and-test.sh
@@ -98,6 +98,11 @@
     bazel build --keep_going $BAZEL_OPTS :install-binaries
     ;;
 
+  compile20)
+    # Compile with C++ 20 to make sure to be compatible with the next version.
+    bazel build --keep_going $BAZEL_OPTS --cxxopt=-std=c++20 :install-binaries
+    ;;
+
   smoke-test)
     $(dirname $0)/smoke-test.sh
     ;;