fix installation script

Author: praiskup

testing-farm/prepare/roles/pre-release-packages/templates/install-pre-release-packages.j2

@@ -2,7 +2,11 @@
 
 set -ex
 
-cd "{{ lookup('env', 'TMT_TREE') }}"
+{% for var in ["TMT_TREE", "PACKIT_PR_ID", "PACKIT_TARGET_SHA", "PACKIT_COMMIT_SHA" %}
+export {{ var }}={{ lookup('env', var ) }}
+{% endfor %}
+
+cd "$TMT_TREE"
 cd "$(git rev-parse --show-toplevel)"
 
 packages=$(PYTHON_PKG_SUFFIX=3 ./releng/detect-changed-packages "$PACKIT_TARGET_SHA")