ci-package-builder: Fix LICENSE_BLACKLIST quotes in dotenv

Storing a dotenv variable using single quotes produces those quotes to be printed when the variable is used.

Because of that, the following statement will always set variable ${LICENSE_BLACKLIST} to a string that includes the enclosing single quotes in its content.

  - echo "LICENSE_BLACKLIST='$LICENSE_BLACKLIST'" | tee -a build.env

Fix this by removing the unnecessary single quotes around value.