Use index .Env for the optional tap and bucket tokens (#50)

The Homebrew cask and Scoop manifest stanzas gated their token and
skip_upload on envOrDefault, which is a sprig function GoReleaser does
not define, so a real release aborted with 'function envOrDefault not
defined' the moment it reached the publish phase. goreleaser check does
not evaluate templates, so the gate slipped through until a tagged run.

Switch to index .Env "NAME", the idiom GoReleaser documents for an
optional variable: it yields the token when the secret is set (push the
update) and an empty string when it is not (skip_upload stays true).

.goreleaser.yaml

@@ -151,11 +151,11 @@ homebrew_casks:
     repository:
       owner: tamnd
       name: homebrew-tap
-      token: '{{ envOrDefault "HOMEBREW_TAP_GITHUB_TOKEN" "" }}'
+      token: '{{ index .Env "HOMEBREW_TAP_GITHUB_TOKEN" }}'
     directory: Casks
     homepage: https://github.com/tamnd/kage
     description: Clone any website for offline viewing, with the JavaScript stripped out
-    skip_upload: '{{ if envOrDefault "HOMEBREW_TAP_GITHUB_TOKEN" "" }}false{{ else }}true{{ end }}'
+    skip_upload: '{{ if index .Env "HOMEBREW_TAP_GITHUB_TOKEN" }}false{{ else }}true{{ end }}'
     commit_author:
       name: Duc-Tam Nguyen
       email: tamnd87@gmail.com
@@ -168,11 +168,11 @@ scoops:
     repository:
       owner: tamnd
       name: scoop-bucket
-      token: '{{ envOrDefault "SCOOP_BUCKET_GITHUB_TOKEN" "" }}'
+      token: '{{ index .Env "SCOOP_BUCKET_GITHUB_TOKEN" }}'
     homepage: https://github.com/tamnd/kage
     description: Clone any website for offline viewing, with the JavaScript stripped out
     license: MIT
-    skip_upload: '{{ if envOrDefault "SCOOP_BUCKET_GITHUB_TOKEN" "" }}false{{ else }}true{{ end }}'
+    skip_upload: '{{ if index .Env "SCOOP_BUCKET_GITHUB_TOKEN" }}false{{ else }}true{{ end }}'
     commit_author:
       name: Duc-Tam Nguyen
       email: tamnd87@gmail.com