Turn back on sweep thread during GC.start #91

Job	Run time
tarball	6m 35s
windows / Windows 2025-vs2026 (check)	2m 11s
non_development / non_development	3m 58s
ubuntu / ubuntu (test-bundler-parallel, ubuntu-22.04)	16m 13s
ubuntu / ubuntu (test-bundler-parallel, ubuntu-24.04)	17m 26s
ubuntu / ubuntu (check, ubuntu-22.04)	4m 2s
ubuntu / ubuntu (test-bundled-gems, ubuntu-22.04)	7m 19s
ubuntu / ubuntu (check, ubuntu-24.04)	4m 37s
macos / macos (test-bundled-gems, macos-15)	9m 19s
macos / macos (test-bundler-parallel, macos-15)	15m 32s
macos / macos (test-bundler-parallel, macos-14)	15m 59s
macos / macos (macos-15-intel, check)	8m 3s
macos / macos (check, macos-15)	4m 26s
macos / macos (test-bundled-gems, macos-14)	6m 51s
macos / macos (check, macos-14)	2m 49s
ubuntu / ubuntu (test-bundled-gems, ubuntu-24.04)	7m 56s
windows / Windows 2022 (check)	2m 10s
	2h 15m 26s

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