fix: use `$JAVA_OPTS` in `decoder4u` #2027

Job	Run time
build (macos-latest)	2m 36s
java_matrix	3s
download_test_data	3s
build (ubuntu-latest)	2m 46s
unit_tests (21)	2m 50s
unit_tests (25)	2m 43s
spotbugs	6m 25s
dependency_analysis	1m 15s
test_clara	4m 27s
test_decoder	3m 0s
test_coatjava (ubuntu-latest, kpp)	3m 10s
test_coatjava (macos-latest, eb-ep, ./run-eb-tests.sh -100 electronproton)	3m 10s
test_coatjava (ubuntu-latest, eb-ep)	1m 55s
test_coatjava (ubuntu-latest, eb-eg)	1m 56s
test_run-groovy	29s
test_coatjava (ubuntu-latest, eb-epc)	2m 15s
test_coatjava (ubuntu-latest, eb-enc)	1m 47s
test_coatjava (ubuntu-latest, eb-eftpi)	1m 56s
release_build	0s
generate_documentation	1m 7s
deploy_web_pages	0s
final	4s
	43m 57s

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