Fix findJava in knox-functions.sh

5 years ago, when I added shellcheck support to our build in the scope of KNOX-1816, I introduced a bug in the findJava function in knox-functions.sh: when $JAVA_HOME is not set, and Java is not available on the path, the function tries to find java executables under /usr. However, the current implementation is wrong:

$ which java
/usr/bin/which: no java in (/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin)
$ echo $JAVA_HOME

$ bin/knoxcli.sh export-cert --type JKS
Warning: JAVA is not set and could not be found.
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