GitHub CI workflow selects dev when building prod branch

[hannes-ucsc]

From a recent promotion:

https://github.com/DataBiosphere/azul/actions/runs/5674220102/job/15377359216#step:4:3

This is why we detected #5426 only after the merge commit had already been pushed, and the build failed on GitLab. It did not fail on GitHub, but should have.

When building the prod branch, or a PR branch based on it, the prod deployment should be selected.

When building the develop branch, or a PR branch based on it, the dev deployment should be selected, as is currently the case.

[hannes-ucsc]

Index: .github/workflows/ci.yml
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
--- a/.github/workflows/ci.yml	(revision 3025429b8a6eb784dc7799340e4c775184109add)
+++ b/.github/workflows/ci.yml	(date 1735347224051)
@@ -24,8 +24,9 @@
 
       - name: 'Run unit tests and other checks'
         run: |
-          source environment
-          _link dev
+          source environment  # load global defaults
+          deployment=$(PYTHONPATH=src python scripts/check_branch.py --print)
+          _link $deployment
           _refresh
 
           make virtualenv

and add support for GITHUB_BASE_REF if set, or GITHUB_HEAD_REF otherwise, to check_branch.py, in addition to GitLab's CI_COMMIT_REF_NAME.

[hannes-ucsc]

For demo, show that

1. when building the prod branch, or a PR branch based on it, the prod deployment is selected.

2. when building the anvilprod branch, or a PR branch based on it, the anvilprod deployment is selected.

3. when building the develop branch, or a PR branch based on it, the dev deployment is be selected.

[achave11-ucsc]

Partially demoed for pull builds but it remains to be demoed for push builds.