std.Build.Step.Run: Add support for LazyPath environment variables

[toivosnare]

Implements #19482

This is a breaking change as there is no longer a directly accessible process.EnvMap.

This makes the addPathDir API little awkward. Currently it just silently returns early if the PATH environment variable is not the string variant. Perhaps EnvVar should have a list variant where each element is either a string or a LazyPath. That would add a lot of complexity though.

Whether environment variables should have all the same variants as command line arguments (addArtifactArg, addPrefixedArtifactArg, addOutputDirectoryArg etc.) should also be discussed.