fetch-docs: allow REPO/REPO_ID/NAME overrides from the environment

[grandixximo]

Make the three source constants overridable from the environment. Defaults are unchanged, so existing cron lines behave as before.

Reason: LinuxCNC/doc-search CI builds linuxcnc-doc-search.tar.gz, the plain docs artifact plus an in-browser pagefind search box (via the topbar slot from LinuxCNC/linuxcnc#4184). It rebuilds only when the docs digest changes, so there is no artifact churn. The trust model is unchanged: head_repository_id pin, sha256 check, tree checks. The webserver token needs nothing new; doc-search is a public repo.

@andypugh: searchable devel docs go live by editing the existing master cron line (a second line on the same webroot would fight it):

FETCH_DOCS_REPO=LinuxCNC/doc-search FETCH_DOCS_REPO_ID=1275651762 \
FETCH_DOCS_NAME=linuxcnc-doc-search.tar.gz \
fetch-docs master /var/www/.../docs/devel

Keep the old line commented for rollback. 2.9 stays on the plain artifact; its pages predate the topbar slot.

[andypugh]

Sorry, but I need a bit more help than this.
The above looks nothing like the existing crontab.

0 * * * * /home/emcboard/fetch-docs 2.9 /home/emcboard/www.linuxcnc.org/docs/stable/ 0 * * * * /home/emcboard/fetch-docs master /home/emcboard/www.linuxcnc.org/docs/devel/

(it doesn't help that crontab -e uses a unique editor that I don't know, JOE)

[grandixximo]

Thanks, that is exactly what I needed. Here is the line in your crontab's shape, one line, no continuations:

0 * * * * FETCH_DOCS_REPO=LinuxCNC/doc-search FETCH_DOCS_REPO_ID=1275651762 FETCH_DOCS_NAME=linuxcnc-doc-search.tar.gz /home/emcboard/fetch-docs master /home/emcboard/www.linuxcnc.org/docs/devel/

It replaces the existing devel line; keep the old one commented above it so rollback is just swapping back.

Two things to know:

• This only takes effect once this PR is merged and you copy the updated fetch-docs over the one at /home/emcboard/fetch-docs. With the script that is there now, the env vars are silently ignored and it keeps publishing the plain docs, so nothing breaks if the line goes in early.
• The hourly cadence is fine. A new searchable artifact appears at most every 6 hours, and only when the docs actually changed, so most runs are no-ops via the stamp file.

For JOE: make the edit, then Ctrl+K followed by X saves and exits; Ctrl+C aborts without saving.