github: fix the comments in test-osbuild-composer-integration

The comments were referring to the merge-base between the PR and main
being used to test the base case when in fact the base is the HEAD of
main at the time the PR was created or updated.

This commit fixes the comments to reflect reality.

.github/workflows/test-osbuild-composer-integration.yml

@@ -8,19 +8,20 @@ name: "osbuild-composer integration"
 # author and reviewers of this change. The workflow works as follows:
 #
 # 1. Checks out osbuild/osbuild-composer
-# 2. Replaces the osbuild/images dependency with the *merge base* of the open
-#    PR with main and runs osbuild-composer's unit tests.
-# 3. If the unit tests on the merge base (step 2) succeed, replaces the
+# 2. Replaces the osbuild/images dependency with the base of the PR (this is
+#    the HEAD of main at the time the PR was opened or updated) and runs
+#    osbuild-composer's unit tests.
+# 3. If the unit tests on the base (step 2) succeed, replaces the
 #    osbuild/images dependency with the *HEAD* of the open PR and runs
-#    osbuild-composer's unit tests. If the tests on the merge base failed, no
-#    further action is taken.
+#    osbuild-composer's unit tests. If the tests on the base failed, no further
+#    action is taken.
 # 4. At most one of two messages is posted:
-# 4.1 Posts a message on the open PR only if the unit tests with the merge base
-#     (step 2) succeed and the unit tests with the PR HEAD (step 3) fail. This
+# 4.1 Posts a message on the open PR only if the unit tests with the base (step
+#     2) succeed and the unit tests with the PR HEAD (step 3) fail. This
 #     combination of outcomes indicates that the PR is the one responsible for
 #     the breakage.
 # 4.2 Updates the existing message on the open PR only if the unit tests with
-#     the merge base (step2) succeed, the unit tests with the PR HEAD (step 3)
+#     the base (step2) succeed, the unit tests with the PR HEAD (step 3)
 #     succeed, and there is already a message posted by this workflow. This is
 #     meant for cases where a PR initially breaks compatibility and then it gets
 #     fixed. No message should be posted on a PR that doesn't affect the
@@ -32,8 +33,8 @@ name: "osbuild-composer integration"
 #    PR. Running on pull_request_target is needed to have access to repository
 #    secrets (Schutzbot's GitHub token).
 # 2. If the unit tests in this repository fail, the integration will fail and
-#    the message will be posted (if the integration is not failing on the merge
-#    base). This will happen even if there's no actual integration issue, so
+#    the message will be posted (if the integration is not also failing on
+#    main). This will happen even if there's no actual integration issue, so
 #    the message can be misleading. However, subsequent runs that fix the issue
 #    will update the message accordingly.
 
@@ -55,8 +56,7 @@ jobs:
     outputs:
       # Define job outputs
       # (see https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/passing-information-between-jobs#example-defining-outputs-for-a-job)
-      # One output for the test with the merge base and one for the test against
-      # the PR
+      # One output for the test with the base and one for the test against the PR
       base_test: ${{ steps.tests-base.outputs.base_test }}
       pr_test: ${{ steps.tests-pr.outputs.pr_test }}
 
@@ -83,15 +83,15 @@ jobs:
       - name: Mark the working directory as safe for git
         run: git config --global --add safe.directory "$(pwd)"
 
-      - name: Update the osbuild/images reference to the merge base with main
+      - name: Update the osbuild/images reference to the base (main)
         env:
-          merge_base_sha: ${{ github.event.pull_request.base.sha }}
+          base_sha: ${{ github.event.pull_request.base.sha }}
         run: |
           cd osbuild-composer
-          go mod edit -replace github.com/osbuild/images=github.com/osbuild/images@$merge_base_sha
+          go mod edit -replace github.com/osbuild/images=github.com/osbuild/images@$base_sha
           ./tools/prepare-source.sh
 
-      - name: Run unit tests (merge base)
+      - name: Run unit tests (main)
         working-directory: osbuild-composer
         id: tests-base
         # This step will not fail if the test fails, but it will write the