Provide a consistent mount point / allow bind mounts on gitlab runners

Problem/Motivation

See background in 🐛 Hardcode the core project ID for fetching artifacts from the gitlab API Needs review .

Both the phpstan and yarn lint (esline, cspell etc.) caches use absolute file paths to validate the cache.

For core committers and scheduled/on commit runs, the build directory is consistently builds/project/drupal but for MRs from anyone else, it is more like builds/long-build-id. The directory name is exposed as CI_PROJECT_DIR on runs.

It looks like the workaround from https://github.com/phpstan/phpstan/issues/8599#issuecomment-2059227168 might work, but to use that we need a consistently-named directory that we can bind mount to.

I'm not sure what the options here are, but opening for discussion.

With all of this working, it would be a considerable CPU + walltime saving for pipelines (it already is if you're a core committer, but gitlab's two-tier approach to MR pipeline tripped me up).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes