An action cannot set its environment or working directory #708

Observed

mcpp::action carries an argument vector and nothing else. An action cannot set an environment variable or choose its working directory.

mcpp-plugins 0.13–0.14 built a helper program (mcpp-deps) only to supply these two things to vcpkg and to cmake -E tar. The helper then reached consumers' manifests as tools = ["mcpp-deps"]. Because it is a host tool, it also paid for #705 and #707: GalTranslPP's emit build-database took 53 minutes, because emit built this tool in full.

0.15.0 (mcpp-community/mcpp-plugins#32) removes the helper. vcpkg happens to accept every setting as a flag (--vcpkg-root, --disable-metrics, --x-buildtrees-root, ...), and CMake work runs as cmake -P over a script the build program writes. Neither route is available to a tool that reads only its environment or its working directory.

Request

Let an action state env entries and a cwd, recorded in the action's fingerprint like its arguments, so a plugin can run such a tool directly without a helper program.