Use an executor specific output directory

[lewisjared]

Description

Refactor to use path fragments when referring to paths that may be used by out-of-process exectors. We can't assume that the paths are the same for the metric exectors and the ref.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
packages/ref-core/src/ref_core/executor/local.py	100.00% <100.00%> (ø)
packages/ref-core/src/ref_core/metrics.py	100.00% <100.00%> (ø)
...s-esmvaltool/src/ref_metrics_esmvaltool/example.py	100.00% <100.00%> (ø)
...metrics-example/src/ref_metrics_example/example.py	100.00% <100.00%> (ø)
packages/ref/src/ref/datasets/cmip6.py	97.40% <100.00%> (ø)
packages/ref/src/ref/models/metric_execution.py	84.31% <100.00%> (+0.31%)	⬆️
packages/ref/src/ref/solver.py	91.17% <ø> (ø)