[ ci ] re 3067: fix CI on main branch

The extra CI jobs introduced in idris-lang#3067 work fine as long as 'main' is not
the checked out branch. This is due to the fetch to a new branch, which
git does (reasonably) not allow when you're trying to fetch 'main' into
a new branch that's also called 'main'. In this case, we should just
`git pull origin main`, which is what the script now (hopefully) does.

.github/workflows/ci-idris2-and-libs.yml

@@ -627,7 +627,14 @@ jobs:
       - name: Build idris2-pack
         run: |
           git config --global --add safe.directory "${PWD}"
-          git fetch "https://github.com/${GITHUB_REPOSITORY}" "${GITHUB_REF}:${GITHUB_REF_NAME}"
+
+          # only fetch to a new branch if we're not on main (otherwise, git complains)
+          if [[ $(git branch --show-current) != 'main' ]]
+          then git fetch "https://github.com/${GITHUB_REPOSITORY}" "${GITHUB_REF}:${GITHUB_REF_NAME}"
+          else git pull origin main
+          fi
+
+          # rebuild pack with the fetched Idris2
           pack install pack
 
   ######################################################################
@@ -663,7 +670,13 @@ jobs:
       - name: Build pack with PR-Idris
         run: |
           git config --global --add safe.directory "${PWD}"
-          git fetch "https://github.com/${GITHUB_REPOSITORY}" "${GITHUB_REF}:${GITHUB_REF_NAME}"
+
+          # only fetch to a new branch if we're not on main (otherwise, git complains)
+          if [[ $(git branch --show-current) != 'main' ]]
+          then git fetch "https://github.com/${GITHUB_REPOSITORY}" "${GITHUB_REF}:${GITHUB_REF_NAME}"
+          else git pull origin main
+          fi
+
           pack install pack
       - name: Build+install idris2-lsp
         run: |