build: use --serial in 'just test_file' #5161

Job	Run time
Diagnostic export (windows-latest, linked)	3m 10s
Diagnostic export (ubuntu-latest, linked)	1m 5s
Diagnostic export (ubuntu-latest, nightly-latest)	59s
Diagnostic export (windows-latest, nightly-latest)	2m 39s
Diagnostic export (macos-latest, linked)	1m 40s
Diagnostic export (macos-latest, nightly-latest)	2m 11s
	11m 44s

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