Prefix project env vars with `PS_`

[philipnickel]

(repo-wide)

Scripts use generic names like INSTALL_DIR, REPO_BASE_URL, BASE_URL that may collide with callers' environments. Decide whether to namespace all project-controlled env vars with PS_ (e.g. PS_INSTALL_DIR, PS_REPO_BASE_URL). Affects every install script and any documentation mentioning overrides.

[zerothi]

Not only is it a good idea, it also tells us which variables we defined, and which are likely existing due to some other reasons.