Repository mapping manifest does not exist in test sandbox

[fmeum]

Description of the bug:

The .repo_mapping file does not appear in test sandboxes.

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

# WORKSPACE
# MODULE.bazel
# pkg/BUILD
sh_test(
  name = "test",
  srcs = ["test.sh"],
)
# pkg/test.sh
#!/usr/bin/env bash

runfiles_dir="$TEST_SRCDIR/../test.runfiles"
if [[ ! -d "$runfiles_dir" ]]; then
  echo "$runfiles_dir does not exist"
  exit 1
fi
repo_mapping="$TEST_SRCDIR/../test.repo_mapping"
if [[ ! -f "$repo_mapping" ]]; then
  echo "$repo_mapping does not exist"
  exit 1
fi

USE_BAZEL_VERSION=last_green bazel test //pkg:test --enable_bzlmod fails
USE_BAZEL_VERSION=last_green bazel run //pkg:test --enable_bzlmod passes
USE_BAZEL_VERSION=last_green bazel test //pkg:test --enable_bzlmod --spawn_strategy=local passes

Which operating system are you running Bazel on?

Linux

What is the output of bazel info release?

HEAD

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

[fmeum]

@Wyverald

[fmeum]

My guess (it really is only a guess) is that we have to explicitly add the repo mapping manifest to the input mapping in SpawnInputExpander for it to show up in sandboxed and remote execution.

[fmeum]

@bazel-io flag

[meteorcloudy]

Fix cherry picked in #16733, removing the potential release blocker label.