dirtools: ensure ActionResult ouputs are posix

[sluongng]

On Windows Executor, the path separator is backslash \.
The action result that we currently creating and consumed by Bazel would
look like this:

"outputFiles": [
  {
    "path": "bazel-out\\k8-fastbuild\\bin\\b.txt",
    "digest": {
      "hash": "..."
    }
  }
],

and Bazel would err with

remote spawn failed: Invalid action cache entry ...: expected output b.txt does not exist.

Ensure that ActionResult always use posix path so it would look like
this instead:

"outputFiles": [
  {
    "path": "bazel-out/x64_windows-fastbuild/bin/b.txt",
    "digest": {
      "hash": "..."
    }
  }
],

[tylerwilliams]

LGTM

Can you please test linux and mac remote-exec in dev as soon as you submit it and it auto-pushes? Want to avoid accidentally breaking anything :)

[sluongng]

Will test in dev and report back here.

[sluongng]

• Linux: https://app.buildbuddy.dev/invocation/f2228403-d1c4-4bf9-bffd-5a3a5a89ef2d?actionDigest=0c1a189263445e9250812a92accbba2d8b0dfdcd99da14ff0fdecf3ff6012d4c%2F166&executeResponseDigest=96b51123ce2f2eb134da11361978e0c0dd084bebe61ef7807f6ad23ee6f0719a%2F120#action

• Mac: https://app.buildbuddy.dev/invocation/90cc8b27-9e09-4870-8424-0d593babd9f1?actionDigest=aa326b9e38d65d1a4397d68468ff0c502ca51edaffbddafbb0be4f3f3d2ff6cc%2F167&executeResponseDigest=22440da83084476b4ded90c7fc25b4cc16ad37f9c19a159bce6b3c67fd083ed7%2F120#action

both worked fine