Support remote build execution on main and read-only remote cache on PRs

[bclark8923]

Description

Please include a summary of the changes and the related issue. Please also
include relevant motivation and context.

Fixes # (issue)

Type of change

Please delete options that aren't relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please also list any relevant details for your test configuration

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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This change is 

[aaronmondal]

Reviewed 1 of 1 files at r5, all commit messages.
Reviewable status: 2 of 1 LGTMs obtained, and all files reviewed, and pending CI: Cargo Dev / macos-13, Installation / macos-13, Installation / macos-14, Remote / large-ubuntu-22.04, nativelink-dot-com-cache-cloud-legacy-dockerfile-test

[adam-singer]

Reviewed 1 of 1 files at r6, all commit messages.
Reviewable status: 2 of 1 LGTMs obtained, and all files reviewed, and pending CI: Remote / large-ubuntu-22.04

[bclark8923]

Reviewed 2 of 2 files at r7, all commit messages.
Reviewable status: complete! 2 of 1 LGTMs obtained, and all files reviewed