CI: let tag '%ci:reset-backlog' will affect PRs too

.cirrus.yml

@@ -33,6 +33,7 @@ build_task:
         # If commit message contains line '%ci:reset-backlog', builds will be done
         # only for current commit.
         unset CIRRUS_LAST_GREEN_CHANGE
+        unset CIRRUS_BASE_SHA
     fi
     MODIFIED_PACKAGES=$(./scripts/build/ci/determine_git_changes.sh)
     for package in $MODIFIED_PACKAGES; do
@@ -72,6 +73,7 @@ publish_task:
   upload_to_bintray_script: |
     if grep -qiP '^\s*%ci:reset-backlog\s*$' <(git log --format="%B" -n 1 "$CIRRUS_CHANGE_IN_REPO"); then
         unset CIRRUS_LAST_GREEN_CHANGE
+        unset CIRRUS_BASE_SHA
     fi
     MODIFIED_PACKAGES=$(./scripts/build/ci/determine_git_changes.sh)
     if [ -n "$MODIFIED_PACKAGES" ]; then