Source build.env manually instead of using gitlab dotenv

It causes the entire pipeline to fail: https://gitlab.haskell.org/ghc/ghcup-ci/-/jobs/1633183

The gitlab upgrade caused artifacts: dotenv: to start failing for some reason

.gitlab/gen_ci.hs

@@ -280,10 +280,7 @@ job arch opsys = (jobName, Job {..})
     jobDockerImage = dockerImage arch opsys
 
     jobScript
-      = [ "echo $METADATA_URI"
-        , "echo $VERSION"
-        ]
-        <> updateRepos opsys <>
+      = updateRepos opsys <>
         [ installToolsScript opsys
         , extraScript arch opsys
         , (if opsys == Windows then "bash " else "") ++  ".gitlab/install-bindist.sh ghc"