build: use --serial in 'just test_file' #7876

Job	Run time
Packaging: Action input (macos-latest, linked)	1m 39s
Packaging: Action input (ubuntu-latest, default)	1m 37s
Packaging: Action input (ubuntu-latest, nightly-latest)	1m 57s
Packaging: Action input (macos-latest, default)	1m 21s
Packaging: Action input (windows-latest, default)	3m 33s
Packaging: Action input (macos-latest, nightly-latest)	1m 53s
Packaging: Action input (windows-latest, nightly-latest)	4m 45s
Packaging: Action input (ubuntu-latest, linked)	1m 33s
Packaging: Action input (windows-latest, linked)	4m 42s
	23m 0s

Provide feedback