[5.0] treat a `end_block_num=0` as "forever" in snapshot scheduler for compatibility with leap 4.0 behavior #6536

Job	Run time
Discover Versions	3s
Platform Cache / Discover Platforms	5s
Platform Cache / Build Platforms (none)	2s
Run Build Workflow / Build leap (reproducible)	7m 45s
Run Build Workflow / Build leap (ubuntu20)	8m 8s
Run Build Workflow / Build leap (ubuntu22)	8m 12s
NP Tests (ubuntu20)	6m 34s
Build deb packages (ubuntu20)	13m 23s
NP Tests (ubuntu22)	5m 11s
Tests (ubuntu20)	3m 27s
Build deb packages (ubuntu22)	12m 14s
NP Tests (ubuntu20repro)	6m 20s
Build deb packages (reproducible)	11m 22s
LR Tests (ubuntu20)	24m 5s
Tests (ubuntu22)	3m 18s
LR Tests (ubuntu22)	24m 5s
Tests (ubuntu20repro)	3m 18s
LR Tests (ubuntu20repro)	24m 4s
Tests (ubuntu22repro)	3m 18s
LR Tests (ubuntu22repro)	23m 57s
NP Tests (ubuntu22repro)	5m 7s
libtester tests (ubuntu20, build-tree)	8m 24s
libtester tests (ubuntu20, make-dev-install)	8m 54s
libtester tests (ubuntu20, deb-install)	4m 17s
libtester tests (ubuntu22, build-tree)	8m 16s
libtester tests (ubuntu22, make-dev-install)	7m 20s
libtester tests (ubuntu22, deb-install)	3m 58s
All Required Tests Passed	1s
	3h 55m 8s

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