build: use --serial in 'just test_file' #5160

Job	Run time
Diagnostic export (ubuntu-latest, linked)	3s
Diagnostic export (macos-latest, linked)	7s
Diagnostic export (ubuntu-latest, nightly-latest)	5s
Diagnostic export (macos-latest, nightly-latest)	8s
Diagnostic export (windows-latest, nightly-latest)	13s
Diagnostic export (windows-latest, linked)	10s
	46s

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