Add env var lookups to gitian.yml

This way we are not asking the user to make local edits to a version
controlled file

gitian.yml

@@ -3,11 +3,11 @@
   become: yes
   hosts: localhost:zcash-build
   vars:
-    zcash_git_repo_url: https://github.com/zcash/zcash
-    zcash_version: v1.0.8-1
+    zcash_git_repo_url: "{{ lookup('env','ZCASH_GIT_REPO_URL') }}"
+    zcash_version: "{{ lookup('env','ZCASH_VERSION') }}"
     gpg_key_name: ''
-    git_name: ''
-    git_email: ''
+    git_name: "{{ lookup('env','GIT_NAME') }}"
+    git_email: "{{ lookup('env','GIT_EMAIL') }}"
     gpg_key_id: ''
   roles:
     - role: common