Allow sharing caches across actions when sandboxing is in use

### Description of the problem / feature request:

Extremely low performance of bazel when building a large project on macOS when compared to running CMake + make.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Build a large bazel project on macOS. I see that a lot of `fsevents` for files in `/private/var/tmp/_bazel_user` triggered and `fseventsd` seems to crawl at 100% - 200% CPU usage after them, slowing down the builds. I do have `/private/var/tmp` masked out in my spotlight configuration, but it does not seem to be enough.

### What operating system are you running Bazel on?

```
Darwin grisu.dyndns.cern.ch 18.2.0 Darwin Kernel Version 18.2.0: Mon Nov 12 20:24:46 PST 2018; root:xnu-4903.231.4~2/RELEASE_X86_64 x86_64
```

### What's the output of `bazel info release`?

```
release 0.22.0
```