Nomad: [enhancement] Update Java driver fingerprinting for macOS

[angrycub]

On versions of macOS without Java installed, there is a stub executable that presents a warning to users that there is no longer a default JDK installed on macOS.

The fingerprinter runs this stub command every 30 seconds while Nomad is running because of
https://github.com/hashicorp/nomad/blob/65f09ed11941f78db20a8ec4bbd1381dad4f83a7/drivers/java/driver.go#L29-L30
https://github.com/hashicorp/nomad/blob/65f09ed11941f78db20a8ec4bbd1381dad4f83a7/drivers/java/utils.go#L11-L27

It appears that we could fingerprint for an installed JDK (that would be seen by this stub) by looking for one or more symlinks in /Library/Java/JavaVirtualMachines. This would definitely improve the getting started experience on macOS.

[vancluever]

@angrycub and I were discussing workarounds here, so I thought I'd post one here that I just applied: you can remove the /usr/bin/java symlink to make this go away. Not too sure if it comes back eventually though (ie: through patches or with the next version of MacOS).

On modern MacOS you need to disable SIP from recovery mode and remount / read-write (mount -u -w /) before you can operate on the symlink. Of course, you can just delete it from recovery mode too.

[angrycub]

I was able to collect enough of the necessary bits to make a null.jdk that can be put in /Library/Java/JavaVirtualMachines that uses symlinks to /usr/bin/false as the target command.
Tested on macOS Catalina 10.15.4.

Unzip and then sudo cp -R null.jdk /Library/Java/JavaVirtualMachines

null.jdk.zip