uacme: do not override production state dir variable

uacme: do not override production state dir variable

With this commit, issue_cert() can be called multiple times alternating
between staging and production certificates within a script.

Before this commit, the production state dir was stored in $STATE_DIR.
But in the case of $use_staging=1, this variable was overwritten in
issue_cert() with $STAGING_STATE_DIR. This made it impossible to call
issue_cert() with $use_staging=0 afterwards. Now the production state
dir is stored in $PRODUCTION_STATE_DIR. This way it is not overridden
anymore and issue_cert() can be called multiple times alternating with
production and staging.

Signed-off-by: Leonardo Mörlein <git@irrelefant.net>