fix: use `$JAVA_OPTS` in `decoder4u` #2612

Job	Run time
validation / GitHub context	3s
validation / Dependency info	5s
validation / Configuration files	9s
validation / Build coatjava	4m 20s
validation / Event generation (e_gFT)	1m 26s
validation / Event generation (e_n)	1m 4s
validation / Event generation (e_g)	1m 17s
validation / Event generation (eFT_K)	1m 18s
validation / Event generation (e_p)	1m 6s
validation / Event generation (e_pC)	1m 5s
validation / Run (e_gFT, rgb_fall2019)	4m 58s
validation / Run (e_pC, rgb_fall2019)	3m 58s
validation / Run (e_p, rgb_fall2019)	4m 15s
validation / Run (e_n, rgb_fall2019)	4m 39s
validation / Run (e_g, rgb_fall2019)	4m 10s
validation / Run (eFT_K, rgb_fall2019)	5m 3s
validation / Report efficiency	4s
validation / Report multiplicity	6s
	39m 6s

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