MANPATH needs a trailing ':' to utilize system defaults (#21682)

otherwise spack breaks using system man pages by default.

Co-authored-by: Harmen Stoppels <harmenstoppels@gmail.com>

var/spack/repos/builtin.mock/packages/mpileaks/package.py

@@ -28,6 +28,7 @@ class Mpileaks(Package):
 
     def install(self, spec, prefix):
         touch(prefix.mpileaks)
+        mkdirp(prefix.man)
 
     def setup_environment(self, senv, renv):
         renv.set('FOOBAR', self.name)