[infra] Use Pub Workspaces #557

Job	Run time
analyze_jni	59s
analyze_jnigen (stable)	42s
build_jni_example_android	4m 3s
build_jni_example_linux	1m 26s
build_jni_example_windows	4m 43s
run_pdfbox_example_linux	2m 39s
test_jnigen_windows_minimal	7m 23s
test_jnigen (ubuntu-latest, stable)	10m 54s
test_jnigen (ubuntu-latest, main)	11m 8s
test_jnigen_macos_minimal	3m 34s
test_jni	1m 52s
test_jni_macos_minimal	1m 24s
test_jni_windows_minimal	4m 41s
coveralls_finish	3s
	55m 31s

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