Bump subosito/flutter-action from 2.12.0 to 2.15.0 #95

Job	Run time
build_jni_example_linux	1m 25s
analyze_jnigen (stable)	1m 0s
analyze_jni	55s
build_jni_example_android	3m 19s
build_jni_example_windows	5m 50s
run_pdfbox_example_linux	2m 39s
test_jni	1m 38s
test_jni_macos_minimal	3m 42s
test_jni_windows_minimal	4m 57s
test_jnigen_macos_minimal	5m 7s
test_jnigen (ubuntu-latest, stable)	8m 22s
test_jnigen_windows_minimal	8m 4s
test_jnigen (ubuntu-latest, beta)	9m 15s
coveralls_finish	3s
	56m 16s

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