Fix tar flags (and filename) on OpenBSD

[sdassow]

This PR addresses #5195 and makes sure the compression is supported by tar in the base system, as well as using the corresponding file name.

Without this patch:

$ fyne package --os openbsd --release
failed to create archive with tar: tar: unknown option -- J
usage: tar {crtux}[014578befHhjLmNOoPpqsvwXZz]
           [blocking-factor | archive | replstr] [-C directory] [-I file]
           [file ...]
       tar {-crtux} [-014578eHhjLmNOoPpqvwXZz] [-b blocking-factor]
           [-C directory] [-f archive] [-I file] [-s replstr] [file ...]

[coveralls]

coverage: 59.953% (-0.008%) from 59.961%
when pulling b42e83c on sdassow:fix-tar-on-openbsd
into de361f6 on fyne-io:develop.