Use envOrDefault for GPG_PRIVATE_KEY_FILE environment variable

By using `envOrDefault` an issue is prevented in evaluating the
template before a release is created. By setting the default value
to `ENV_VAR_GPG_PRIVATE_KEY_FILE_NOT_SET`, this value is then shown
in the error message at signing time.

The reason for changing this is so that it becomes possible to
create local, unsigned releases when creating a release as follows:

 `goreleaser release --snapshot --clean --skip sign,after`

Author: hslatman

.goreleaser.yml

@@ -133,10 +133,10 @@ nfpms:
       postremove: scripts/postremove.sh
     rpm:
       signature:
-          key_file: "{{ .Env.GPG_PRIVATE_KEY_FILE }}"
+          key_file: '{{ envOrDefault "GPG_PRIVATE_KEY_FILE" "ENV_VAR_GPG_PRIVATE_KEY_FILE_NOT_SET" }}'
     deb:
       signature:
-          key_file: "{{ .Env.GPG_PRIVATE_KEY_FILE }}"
+          key_file: '{{ envOrDefault "GPG_PRIVATE_KEY_FILE" "ENV_VAR_GPG_PRIVATE_KEY_FILE_NOT_SET" }}'
           type: origin
   -
     << : *NFPM