Add build_type to datasource github_repository's pages (#1710)

PR #1663 added the "build_type" attribute to the github_repository resource.
Since the corresponding data source uses the same code to parse the pages
endpoint data, any use of the github_repository datasource would lead to an
error, since the attribute did not exist there:

> Error: error setting pages: Invalid address to set: []string{"pages", "0", "build_type"}

This commit adds the missing attribute. The existing acceptance tests already
cover it, failed for me when running without this commit, and passed after.

Co-authored-by: Keegan Campbell <me@kfcampbell.com>

github/data_source_github_repository.go

@@ -135,6 +135,10 @@ func dataSourceGithubRepository() *schema.Resource {
 								},
 							},
 						},
+						"build_type": {
+							Type:     schema.TypeString,
+							Computed: true,
+						},
 						"cname": {
 							Type:     schema.TypeString,
 							Computed: true,