Backport support of APP_CACHE_DIR

[vrobert78]

1. Why is this change necessary?

We want to separate the cache directory from the /var folder on Platform.sh as /var is a NFS mount and cache a local one, and it triggers caching issues.
It was done for the 6.6, but it would be great to have it on the 6.5 too, hence this backport.

2. What does this change do, exactly?

It gives the possibility to define a new cache folder via APP_CACHE_DIR.

3. Describe each step to reproduce the issue or behaviour.

4. Please link to the relevant issues (if any).

See 9997eb7

5. Checklist

• I have rebased my changes to remove merge conflicts
• I have written tests and verified that they fail without my change
• I have created a changelog file with all necessary information about my changes
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

	Fails
🚫	The title `Backport support of APP_CACHE_DIR` does not match our requirements. Example: NEXT-00000 - My Title

	Warnings
⚠️	The Pull Request doesn't contain any changelog file

[AydinHassan]

hi @vrobert78 can you sign the CLA please :)

[shyim]

I cherry-picked the PR internally from the trunk, it will be soon merged :) and hope to release it next week