Provide fallback for GovukAdminTemplate.environment_label

It turns out that the GOVUK_ENVIRONMENT_NAME env var is not defined and
the Rails env is not development or test during the GH action job which
builds the image for deployment. This means that GovukEnvironment.name
is nil and we see the following exception [1]:

    NoMethodError: undefined method `titleize' for nil:NilClass

I considered setting GOVUK_ENVIRONMENT_NAME in the GH action, but there
didn't seem to be any precendent for doing that.

I also considered providing a fallback within the implementation of
GovukEnvironment.name. However, I thought that might have other
undesirable consequences.

So I've implemented the fallback in
config/initializers/govuk_admin_template.rb which is a bit ugly, but
hopefully it won't be too long before we get rid of GovukAdminTemplate
altogether!

[1]: https://github.com/alphagov/signon/actions/runs/5940071624/job/16107859623#step:7:609

config/initializers/govuk_admin_template.rb

@@ -8,5 +8,5 @@
   c.disable_google_analytics = false
 end
 
-GovukAdminTemplate.environment_label = GovukEnvironment.name.titleize
+GovukAdminTemplate.environment_label = (GovukEnvironment.name || "development").titleize
 GovukAdminTemplate.environment_style = GovukEnvironment.production? ? "production" : "preview"