Use jsonnet.EvaluateAnonymousSnippet

[tombrk]

Jsonnet recently introduced a new function class for evaluating snippets that are not actually files on disk. google/go-jsonnet#447

We have several places this applies, so we should adopt this:

mixtool/pkg/mixer/eval.go

Line 25 in f4e131d

return vm.EvaluateSnippet("", snippet)

mixtool/pkg/mixer/eval.go

Line 30 in f4e131d

return vm.EvaluateSnippet("", snippet)

mixtool/pkg/mixer/eval.go

Line 35 in f4e131d

return vm.EvaluateSnippet("", snippet)