Enhancement: Add the ability to extend CAJA_EXTENSIONDIR with environment variables

[ghost]

I propose to you to add the ability to control the CAJA_EXTENSIONDIR with environment variables.
The Package manager and GNU Distribution I work with (and package Mate for) can programatically control the path every application is aware of.
As CAJA_EXTENSIONDIR is currently a macro that is recorded in the build-process, we (probably Nix/NixOS aswell) would prefer a way to set the directory caja looks into for extensions (forming a union of multiple paths).
As this has been a counter-question in the past: just XDG_PATH doesn't help us (for various reasons).

We prefer to keep as little downstream patches as possible. If it helps, for Pidgin Nix and Guix proceeded like this: https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/patches/pidgin-add-search-path.patch
In the case of pidgin-otr (plugin) this allows us to use PURPLE_PLUGIN_PATH for pidgin plugins provided by the package manager in a reproducible manner.

Resolving this request would effectively fix multiple other software in Mate Desktop on GNU Guix / GNU GuixSD, all which set and/or detect the "caja path" in the build-process to install caja plugins.

Expected behaviour

Actual behaviour

Steps to reproduce the behaviour

MATE general version

1.18.x

Package version

1.18.3

Linux Distribution

GNU GuixSD

Link to downstream report of your Distribution

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