Use less fetchers in overnight build to work-around limitations from servers.

Description

Issue happens with sourceware.org server.

See for example job https://gitlab.com/BuildStream/buildstream/-/jobs/114575697

This merge request, when approved, will close: Fixes #742 (closed)

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mentioned in issue #742 (closed)

changed the description

This should mitigate the issue (there is still a chance 2 different jobs access at the same time the same repo, but I think the probability is quite low).

Nevertheless this will improve the current situation

approved this merge request

enabled an automatic merge when the pipeline for 62942bfd succeeds

merged

mentioned in commit 442da2f9