Limit max-parallel of per-suite task to 4

[vicamo]

In the hope to prevent following intermittent errors:

+ wget -O /tmp/debuerreotype.sid.2wg1ZPx2tI/output/20250125/sh4/sid/InRelease http://snapshot.debian.org/archive/debian-ports/20250125T033450Z/dists/sid/InRelease
--2025-01-26 03:34:56--  http://snapshot.debian.org/archive/debian-ports/20250125T033450Z/dists/sid/InRelease
Resolving snapshot.debian.org (snapshot.debian.org)... 146.75.30.132, 2a04:4e42:77::644
Connecting to snapshot.debian.org (snapshot.debian.org)|146.75.30.132|:80... connected.
HTTP request sent, awaiting response... 503 first byte timeout
2025-01-26 03:35:12 ERROR 503: first byte timeout.

+ '[' -f /tmp/debuerreotype.sid.2wg1ZPx2tI/debian-archive-sid-keyring.gpg ']'
+ wget -O /tmp/debuerreotype.sid.2wg1ZPx2tI/output/20250125/sh4/sid/Release.gpg http://snapshot.debian.org/archive/debian-ports/20250125T033450Z/dists/sid/Release.gpg
--2025-01-26 03:35:12--  http://snapshot.debian.org/archive/debian-ports/20250125T033450Z/dists/sid/Release.gpg
Resolving snapshot.debian.org (snapshot.debian.org)... 146.75.30.132, 2a04:4e42:77::644
Connecting to snapshot.debian.org (snapshot.debian.org)|146.75.30.132|:80... connected.
HTTP request sent, awaiting response... 503 first byte timeout
2025-01-26 03:35:28 ERROR 503: first byte timeout.

+ '[' sid = slink ']'
+ echo 'error: failed to fetch either InRelease or Release.gpg+Release for '\''sid'\'' (from '\''http://snapshot.debian.org/archive/debian-ports/20250125T033450Z'\'')'
error: failed to fetch either InRelease or Release.gpg+Release for 'sid' (from 'http://snapshot.debian.org/archive/debian-ports/20250125T033450Z')
+ exit 1