(20969) Use colon for path separator on all platforms

I had this backwards in 3d510e4b6f and inadvertly pushed for the two
to be normalized in 5966907. This is needed because Cygwin systems
interpret the PATH env var (and require a colon as a separator) prior
to passing it on to Windows while other env vars are passed directly
to the OS.

Signed-off-by: Justin Stoller <justin@puppetlabs.com>