RFC Bazel: Fallback to sandboxed spawn strategy if remote not supported

Fallback to sandboxed spawn strategy if remote spawn strategy is not
supported, as suggested in [1]:

If you currently use remote execution with --strategy=remote and/or
--spawn_strategy=remote and have actions that might not be executed
remotely, consider removing those strategy flags completely, in this
case bazel will pickup the first available strategy from the default
list, which is remote,worker,sandboxed,local. Alternatively, add a
strategy to fallback to if remote is not possible for an action, for
example --spawn_strategy=remote,sandboxed will fallback to a sandboxed
execution if remote is not possible.

[1] bazelbuild/bazel#7480

Bug: Issue 12356
Change-Id: Ibbf7365afe647bbb77239b00345cbb801141d45c

tools/remote-bazelrc

@@ -57,7 +57,7 @@ build:remote --platforms=@bazel_toolchains//configs/ubuntu16_04_clang/1.2:rbe_ub
 # Set various strategies so that all actions execute remotely. Mixing remote
 # and local execution will lead to errors unless the toolchain and remote
 # machine exactly match the host machine.
-build:remote --spawn_strategy=remote
+build:remote --spawn_strategy=remote,sandboxed
 build:remote --strategy=Javac=remote
 build:remote --strategy=Closure=remote
 build:remote --strategy=Genrule=remote