chore(wrapper/publish) serve uctest.json and the HTML in the virtual tree download/ from Apache instead of mirrors

[dduportal]

Related to jenkins-infra/helpdesk#4311 (comment) (second bullet).

Closes 810 (as not needed anymore)

This PR is a follow up of #812 for the new Update Center using Apache2 + Mirrorbits.

It updates the publication script to serve the following file from Apache instead of mirrors:

• uctest.json => solves the HTTP/404 on azure.updates.jenkins.io/whatever?uctest
• download/**/*.html (only these files are present: other are sent to get.jenkins.io` => should solve the broken links in these HTML pages

As #812 did add a condition on "the file must not exist in httpd documentroot" to redirect incoming requests to the UC mirrors, Apache will serve these files.

As discussed with @daniel-beck:

• These files will count in the (billed) outbound bandwidth as served by Apache, but the amount of traffic is really low so it does not even count
• These files won't benefit from the new "mirror proximity" feature. But we do not really care as it is a really low usage
• We won't need to define an absolute URL (constraining us for years to a scheme and an hostname: not really flexible)

[daniel-beck]

Closes #810 (as not needed anymore)

Sure, but as we've discussed, it's a good idea anyway to link to the canonical URLs.

[dduportal]

Closes #810 (as not needed anymore)

Sure, but as we've discussed, it's a good idea anyway to link to the canonical URLs.

Updated the body then, to avoid PR being closed

[dduportal]

Merging as it's only scoped to the publish.sh script to solve the new UC remaining errors