[MapServer-dev] Conda-based CI builds

[Howard Butler]

Devs,

I have made a PR that adapts some of the CI logic we use for PDAL and other projects to build out MapServer using a Conda Forge dependency stack for OSX, Windows, and Linux.

https://github.com/MapServer/MapServer/pull/7102

It doesn't currently implement MapScript or run any tests, but it does exercise the build on all three platforms.

Howard