Revert "Only load sphinx_markdown_builder extension if it's used"

sphinx-build fails with

	sphinx.errors.SphinxError: Builder name markdown not registered or available through entry point

Apparently this issue was hidden locally by caching, and not checked
in CI because of this error causing
tests/checks/sphinx-markdown-changelog.fish to be skipped.

	sphinx-build 7.2.6
	runner@runnervm3ublj:~/work/fish-shell/fish-shell$ python -c 'import sphinx_markdown_builder'
	Traceback (most recent call last):
	  File "<string>", line 1, in <module>
	  File "/home/runner/.local/lib/python3.12/site-packages/sphinx_markdown_builder/__init__.py", line 6, in <module>
	    from sphinx.util.typing import ExtensionMetadata
	ImportError: cannot import name 'ExtensionMetadata' from 'sphinx.util.typing' (/usr/lib/python3/dist-packages/sphinx/util/typing.py)

This reverts commit 7b495497d7.

While at it, fail the test earlier if something went wrong, because the
remaining check will likely also fail and confuse.

(cherry picked from commit aab22a453b)

.github/actions/install-sphinx-markdown-builder/action.yml

@@ -6,7 +6,9 @@ permissions:
 runs:
   using: "composite"
   steps:
-    - run: |
-        commit=a6dd85e7b13107172c0502937bec6b718002510d
+    - shell: bash
+      run: |
+        set -x
+        commit=b259de1dc97573a71470a1d71c3d83535934136b
         pip install git+https://github.com/krobelus/sphinx-markdown-builder@"$commit"
-      shell: bash
+        python -c 'import sphinx_markdown_builder'

doc_src/conf.py

@@ -16,6 +16,10 @@ from docutils import nodes
 
 try:
     import sphinx_markdown_builder
+
+    extensions = [
+        "sphinx_markdown_builder",
+    ]
 except ImportError:
     pass
 

tests/checks/sphinx-markdown-changelog.fish

@@ -7,7 +7,10 @@ if not test -e $workspace_root/.git
     return
 end
 
-$workspace_root/build_tools/release-notes.sh -q >relnotes.md
-or echo "Failed to build Markdown release notes."
+$workspace_root/build_tools/release-notes.sh -q >?relnotes.md
+or {
+    echo "Failed to build Markdown release notes."
+    return
+}
 sed -n 1p relnotes.md | grep -q '^## fish \S* (released .*)'
 or echo "Unexpected changelog title"