FortranDependency: make sure gfortran ends up in PATH

Per requirements.rb:

    >  XXX If the satisfy block returns a Pathname, then make sure that it
    >  remains available on the PATH. This makes requirements like
    >    satisfy { which("executable") }
    >  work, even under superenv where "executable" wouldn't normally be on the
    >  PATH.
    >  This is undocumented magic and it should be removed, but we need to add
    >  a way to declare path-based requirements that work with superenv first.

Fixes homebrew/homebrew-python#170.

Closes Homebrew/homebrew#38448.

Library/Homebrew/requirements/fortran_dependency.rb

@@ -8,6 +8,6 @@ class FortranDependency < Requirement
   env { ENV.fortran }
 
   satisfy :build_env => false do
-    (ENV['FC'] || which('gfortran')) ? true : false
+    which(ENV["FC"] || "gfortran")
   end
 end