Linux tar should not be nested

[alanocallaghan]

My fault for trying to write a valid tar from memory but the current linux build ends up as a tar.xz that contains the build/dist/QuPath directory, rather than the top level being QuPath

[alanocallaghan]

To explain the flags:

-c says create an archive
-C says cd to this directory first
QuPath specifies that after cd-ing, archive the QuPath dir
then we pipe into xz and redirect stdout to the tar.xz file