[plan-build.ps1] Use `RUNTIME_ENVIRONMENT` if necessary for run path.
[plan-build.ps1] Use `RUNTIME_ENVIRONMENT` if necessary for run path.

This change adds one more backwards compatible case when building
Windows Habitat packages. Older packages have their `PATH` entries
exclusively in `RUNTIME_ENVIRONMENT` metadata files and so we need to
fallback and check these entries if no `PATH` metadata file is present
in dependent packages when computing the runtime path value.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat