WIP: cmake: Teach '-E env' to load environment variables from a file

Allow passing files containing environment variables in KEY=VALUE format to the cmake -E command.

Example: settings.env

DEBUG=true
MY_PATH=/path/to/thing

Then you would call:

cmake -E env THIS=that --from-file-v1=settings.env bash -c 'env'

Discussion: https://discourse.cmake.org/t/feature-request-add-env-file-option-to-cmake-command-mode/2344/3

Topic-rename: cmake-E-env-file