get.sh: use standard syntax for locating `javac`

[sxa]

Fixes https://github.com/adoptium/infrastructure/issues/2695

I think this should be equivalent to the old code and work properly on a find command that does not have the -path parameter so will be more portable.

Original failing job listed in the PR: See https://ci.adoptopenjdk.net/job/Grinder/5391 Re-run against this PR branch: See https://ci.adoptopenjdk.net/job/Grinder/5393

If reviews can verify that it looks like it's done the right thing in that job it would be appreciated :-)

Signed-off-by: Stewart X Addison [email protected]