Check for the existence of the file that could be a POSIX filepath

Under cygwin, if you pass a path like /c/cygdrive/, we check that
/c exists and is a directory.

If you have files in the root of the cygwin environment and you pass
/ROOTFILE, we need to weaken the check so that we only check that
the file exists. We don't care that it's not a directory.

(cherry picked from commit 0717c66)

subprojects/plugins/src/main/resources/org/gradle/api/internal/plugins/unixStartScript.txt

@@ -210,7 +210,7 @@ if "\$cygwin" || "\$msys" ; then
             case \$arg in                                #(
               -*)   false ;;                            # don't mess with options #(
               /?*)  t=\${arg#/} t=/\${t%%/*}              # looks like a POSIX filepath
-                    [ -d "\$t" ] ;;                      #(
+                    [ -e "\$t" ] ;;                      #(
               *)    false ;;
             esac
         then