cirrus: Pin dotenv to semver ~> 2.8, not exact 2.8.1

Better to receive future minor/patch updates than pin exact,
considering this dependency used to be totally unbounded ">= 0".

We probably won't remember to update this in the future,
so best to specify a range rather than an exact arbitrary version.

.cirrus.yml

@@ -85,7 +85,7 @@ arm_linux_task:
                 libasound2-dev
                 libnss3
                 xvfb
-    - gem install dotenv -v 2.8.1
+    - gem install dotenv -v '~>  2.8'
     - gem install fpm
     - git submodule init
     - git submodule update