installer/environment.pm: fix variable name

Fixes "Use of uninitialized value in concatenation (.) or string
at solenv/bin/modules/installer/environment.pm line 110."

Since: 38e6216cfd04f23ca0dbfa6bb99c5631f4891e78

Change-Id: Ia4ed62f64b07d1d244e172dd6e02010d896b845b
Reviewed-on: https://gerrit.libreoffice.org/10480
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>