Update go-cty to pull in more stdlib funcs.

I needed "split" specifically so I can do something like:

```hcl
variable PLATFORMS {
  default = "linux/amd64"
}

target foo {
  platforms = split(",", "${PLATFORMS}")
  # other stuff
}
```

Where the existing "csvdecode" does not work for this because it parses
the string into a list of objects instead of a list of strings.

I went ahead and just added all the available new functions.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

vendor/github.com/zclconf/go-cty/cty/marks.go

@@ -200,7 +200,7 @@ func (val Value) Unmark() (Value, ValueMarks) {
 func (val Value) UnmarkDeep() (Value, ValueMarks) {
 	marks := make(ValueMarks)
 	ret, _ := Transform(val, func(_ Path, v Value) (Value, error) {
-		unmarkedV, valueMarks := val.Unmark()
+		unmarkedV, valueMarks := v.Unmark()
 		for m, s := range valueMarks {
 			marks[m] = s
 		}