Remove bad interpreted --parallel flag for builds #15030

Job	Run time
Core Basic (18.x)	6m 49s
Core Basic (20.x)	7m 7s
Core Basic - Plone 5 (18.x)	7m 12s
Core Blocks (18.x)	8m 14s
Core Blocks - Listing (18.x)	5m 15s
Core Blocks - Listing (20.x)	5m 4s
Core Blocks (20.x)	8m 1s
Core Content (18.x)	4m 2s
Core Control Panels (18.x)	4m 55s
Core Content (20.x)	4m 17s
Core Volto Slate (18.x)	7m 40s
Core Control Panels (20.x)	4m 29s
Core Volto Slate (20.x)	7m 33s
Coresandbox (18.x)	4m 47s
Project Generator (18.x)	7m 18s
Project Generator (20.x)	7m 25s
Multilingual (18.x)	3m 51s
Multilingual in Seamless Mode (18.x)	3m 57s
Seamless Mode (18.x)	6m 57s
Working Copy (18.x)	4m 16s
	1h 59m 9s

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