Make find_javahome() work with OpenJDK 11 on Ubuntu 20.04

[ehrenfeu]

On Ubuntu 20.04 the find_javahome() function doesn't work as expected as it is cutting away too much of the path.

To make it work, I had to use the same line that is already there for Oracle's Java distribution.

Context:

dpkg --get-selections | grep -i openjdk-11

openjdk-11-jdk-headless:amd64			install
openjdk-11-jre:amd64				install
openjdk-11-jre-headless:amd64			install

And

lsb_release -a

No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04.2 LTS
Release:	20.04
Codename:	focal

[ehrenfeu]

Please feel free to close this PR if you're fixing it differently. This is mostly to let you know what I did to fix it on my side!

[ehrenfeu]

Hi all, maybe @bethac07 ?

I just had to help a few people going through the installation on Ubuntu again and hence would like to bump this PR, as (naturally) they were trying to install from the official repo.

Is there a reason why it shouldn't be merged at this point? Does it break installation on other systems? I'm happy to fix this further, but I'd love to see support for current Ubuntu LTS in upstream.

Thanks ❤️ Niko

[ehrenfeu]

(Just Cc'ing my colleagues @lguerard @cellkai and @sebherbert to keep them in the loop)