fix: use `$JAVA_OPTS` in `decoder4u` #2615

Job	Run time
validation / Dependency info	15s
validation / GitHub context	2s
validation / Configuration files	8s
validation / Build coatjava	2m 16s
validation / Event generation (eFT_K)	1m 4s
validation / Event generation (e_g)	2m 3s
validation / Event generation (e_gFT)	1m 6s
validation / Event generation (e_p)	1m 21s
validation / Event generation (e_n)	1m 43s
validation / Event generation (e_pC)	1m 6s
validation / Run (e_n, rgb_fall2019)	3m 54s
validation / Run (e_pC, rgb_fall2019)	4m 11s
validation / Run (e_p, rgb_fall2019)	4m 51s
validation / Run (eFT_K, rgb_fall2019)	4m 39s
validation / Run (e_g, rgb_fall2019)	4m 2s
validation / Run (e_gFT, rgb_fall2019)	4m 42s
validation / Report multiplicity	4s
validation / Report efficiency	3s
	37m 30s

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