kokoro: Upgrade to GCC 10

GCC 9 appears to emit false-positive warnings about fields being
uninitialized.

Change-Id: I248e087e45c61e04d3cbd626e00501b143ebaf8f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/108922
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

infra/kokoro/linux/docker.sh

@@ -137,7 +137,7 @@ if [ "$BUILD_SYSTEM" == "cmake" ]; then
         COMMON_CMAKE_FLAGS+=" -DTINT_BUILD_AST_FUZZER=1"
         COMMON_CMAKE_FLAGS+=" -DTINT_BUILD_REGEX_FUZZER=1"
     elif [ "$BUILD_TOOLCHAIN" == "gcc" ]; then
-        using gcc-9
+        using gcc-10
     fi
 
     if [ "$BUILD_SANITIZER" == "asan" ]; then