Use File::PATH_SEPARATOR globally instead of ':'

On Unix, the path separator is ':', whereas on Windows,
it is ';'. This is the first of a series of patch to bring
macbrew's and winbrew's codebases closer together.

The main places the magic constant ':' was being used were:
  - the $PATH environment variable
  - CMAKE-related environment variables
  - pkg-config related environment variables

Closes #21921.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>