TreeArtifacts output directories aren't created with sandboxing enabled

[cushon]

With sandboxing enabled, Bazel doesn't create output directories for TreeArtifacts, and tools have to create them manually. The output directories should be created for consistency with other spawn strategies.

This issue was the underlying problem in #6203.

Repro:

• build bazel @ e856d82

BUILD

load(":def.bzl", "r")

r(name = "a")

def.bzl

def _r(ctx):
    d = ctx.actions.declare_directory("%s_dir" % ctx.label.name)
    ctx.actions.run_shell(
        outputs = [d],
        command = "cd %s && pwd" % d.path,
    )
    return [DefaultInfo(files = depset([d]))]

r = rule(implementation = _r)

Building with sandboxing enabled fails, because the directory for the TreeArtifact isn't created:

$ bazel build ...
...
ERROR: /tmp/tmp.wido4a1xyh/BUILD:3:1: error executing shell command: '/bin/bash -c cd bazel-out/k8-fastbuild/bin/a_dir && pwd' failed (Exit 1) bash failed: error executing command /bin/bash -c 'cd bazel-out/k8-fastbuild/bin/a_dir && pwd'

Use --sandbox_debug to see verbose messages from the sandbox

Building with --spawn_strategy=standalone succeeds:

bazel build --spawn_strategy=standalone ...
... OK