CI: properly enable the string dtype also for custom CI builds
Small follow-up on https://github.com/pandas-dev/pandas/pull/61722, where I forgot to set the default of the env variable to 1 if not specified in the matrix (I included it in the main matrix, but then all custom builds that were explicitly listed in the `include: ` section of the matrix would not have the variable, and therefore still default to turn off the string dtype)
Thanks @jorisvandenbossche 