The shared cache misses when the build directory is named by its absolute path

Expected Behavior

_build and $PWD/_build name the same directory, so an entry the shared cache stored under one spelling is restored under the other.

Actual Behavior

The action runs again: the two spellings key the same workspace twice and get separate cache namespaces.

Reproduction

mkdir -p /tmp/dune-bd && cd /tmp/dune-bd
export DUNE_CACHE=enabled DUNE_CACHE_ROOT=/tmp/dune-bd-cache
echo '(lang dune 3.24)' > dune-project
printf '(rule (deps source) (targets target)\n (action (system "cat source > target")))\n' > dune
echo hello > source

dune build --display=short target                             # runs the action: "sh target"
rm -rf _build
dune build --display=short target                             # cache hit: prints nothing
rm -rf _build
DUNE_BUILD_DIR=$PWD/_build dune build --display=short target  # prints "sh target" again

The third command should print nothing, like the second one. --build-dir=$PWD/_build behaves the same as DUNE_BUILD_DIR.

Specifications

• Version of dune: 3.24.2-1011-g7b0f4fb (main at 7b0f4fbd98, bootstrapped)
• Version of ocaml: 5.4.1
• Operating system: macOS 27.0, arm64

Additional information

The build directory is recorded exactly as named (Path.Build.set_build_dir in bin/common.ml:1252), and every rule renders its targets and dependencies from it, so the spelling reaches the rule digest (the sandbox path under --verbose differs between the two spellings). Recording a build directory that sits directly under the workspace root by its relative name turns the third command into a cache hit; I have that patch and a blackbox test and can open a PR.

Start at Path.Build.set_build_dir in bin/common.ml:1252 and trace how the build-directory spelling reaches rule digests and cache keys. Use the reproduction to compare relative and absolute build-directory forms, then add or run the blackbox test mentioned in the issue. Done means the absolute-path command restores the existing shared-cache entry without rerunning the action.