ci: remove parallelization of external deps tests #789

Job	Run time
Build & test (stable, ubuntu-latest)	2m 11s
Build & test (stable, macos-latest)	1m 50s
Build & test (stable, windows-latest)	3m 24s
Build & test (nightly, ubuntu-latest)	4m 29s
Build & test (nightly, macos-latest)	4m 24s
Build & test (nightly, windows-latest)	9m 1s
Clippy lint checks	26s
Test coverage checks	4m 27s
Build with each feature combination	2m 29s
Test with external dependencies	3m 7s
Minimal dependency versions build	1m 19s
Machete dependencies checks	15s
Miri test	8m 10s
Code formatting checks	19s
	45m 51s

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