Chg UBC-MOAD/gha-workflows/ workflow pins to @main

If I can't trust myself, who can I trust?

But really, less maintenance when I have to update 3rd party action pins
in our reusable workflows.

.github/workflows/sphinx-linkcheck.yaml

@@ -16,7 +16,7 @@ jobs:
         # Need to specify Python version here because we use test env which gets its
         # Python version via matrix
         python-version: [ '3.11' ]
-    uses: UBC-MOAD/gha-workflows/.github/workflows/sphinx-linkcheck.yaml@a34def527ceee8686a208588e899f9837085215c
+    uses: UBC-MOAD/gha-workflows/.github/workflows/sphinx-linkcheck.yaml@amain
     with:
       python-version: ${{ matrix.python-version }}
       conda-env-file: environment.yaml