Nextflow launch script: improving search for JAVA_CMD

[marcodelapierre]

This PR aims to address #4653, and as such applies a few small edits to the main nextflow script:

• if no installation found at all (`$JAVA_CMD --version failing), print additional advice (in red):

  NOTE: please refer to the Nextflow Get Started page at https://www.nextflow.io/docs/latest/getstarted.html.
  

• if SDKMAN sourceable script found but not sourced, have Nextflow source it in case it has Java installed; if Java found this way, also print a notice (in yellow):

  NOTE: Nextflow has found a working SDKMAN Java installation and is going to use it. For a more robust setup, add the following lines at the end of your Bash .profile or .bash_profile:
   export SDKMAN_DIR="$HOME/.sdkman"
   [[ -s "$HOME/.sdkman/bin/sdkman-init.sh" ]] && source "$HOME/.sdkman/bin/sdkman-init.sh"
  

  If Java not found, silently fallback to JAVA_CMD=java
• if /usr/libexec/java_home is found but not working, silently fallback to JAVA_CMD=java, for a less confusing error (I think that path search only works on a Mac and if a system Java has been installed)

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[pditommaso]

I would not bind so strictly to SDKman. It would be preferable just report that JAVA_HOME is not defined when it fail to find a java launcher

[marcodelapierre]

Have a look at this iteration:
1- removed auto sourcing of SDKMAN installation
2- message for Java not found more informative:

NOTE: Nextflow needs a Java virtual machine to run. To this end:   
 - make sure a `java` command can be found; or   
 - manually define the variables NXF_JAVA_HOME, JAVA_HOME or JAVA_CMD to point to an existing installation; or    
 - install a Java virtual machine, for instance through https://sdkman.io (read the docs);   
 - for more details please refer to the Nextflow Get Started page at https://www.nextflow.io/docs/latest/getstarted.html.   

[jason-seqera]

@marcodelapierre thanks!