fix: allow `run-groovy` users to set `JAVA_OPTS`

[c-dilks]

libexec/env.sh hard codes a value of $JAVA_OPTS for groovy calls, (via run-groovy) making it impossible for run-groovy users to change any Java options, e.g., heap size limits. This PR allows users to override env.sh's settings by appending user-level JAVA_OPTS.

[c-dilks]

So for example, now this will work

JAVA_OPTS="-Xms256m -Xmx256m" run-groovy .....

[baltzell]

We should reassess what this "run-groovy" script is supposed to do.

[c-dilks]

We should reassess what this "run-groovy" script is supposed to do.

• JYPATH=..... => this is the classpath to include coatjava, and is the primary reason we have run-groovy (cf. run-jshell)
• -Dsun.java2d.pmoffscreen=false => not sure why we use this; seen also in dclayereffs-ana
• -Djava.util.logging.config.file=$CLAS12DIR/etc/logging/debug.properties => somewhat common in wrapper scripts, seems useful
• -Xms1024m -Xmx2048m => in principle we could remove these and trust the defaults, but this PR allows the caller to override them; probably better to keep these here so users aren't surprised by 'out of memory' errors on slurm
• -XX:+UseSerialGC => looks like all our java wrappers in bin/ use this (default is parallel GC)