Checkout correct commit on pull_request in CI

By default, the `pull_request` event has a `GITHUB_SHA` env variable set to the
"last merge commit on the GITHUB_REF branch"
(see https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request).
But we want to check out the latest commit on the branch whether or not it is a
merge commit. This commit changes the CI actions to do just that.

Author: FinleyMcIlwaine

.github/workflows/bench.yml

@@ -57,6 +57,14 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+      with:
+
+        # By default, the `pull_request` event has a `GITHUB_SHA` env variable
+        # set to the "last merge commit on the GITHUB_REF branch" (see
+        # https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request).
+        # But we want to check out the latest commit on the branch whether or
+        # not it is a merge commit, so this is how we do that.
+        ref: "${{ github.event.pull_request.head.sha }}"
 
     - run: git fetch origin master # check the master branch for benchmarking
 

.github/workflows/flags.yml

@@ -23,6 +23,15 @@ jobs:
     steps:
       # Need the repo checked out in order to read the file
       - uses: actions/checkout@v3
+        with:
+
+          # By default, the `pull_request` event has a `GITHUB_SHA` env variable
+          # set to the "last merge commit on the GITHUB_REF branch" (see
+          # https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request).
+          # But we want to check out the latest commit on the branch whether or
+          # not it is a merge commit, so this is how we do that.
+          ref: "${{ github.event.pull_request.head.sha }}"
+
       - id: ghcs
         run: echo "ghcs=$(cat ./.github/workflows/supported-ghc-versions.json)" >> $GITHUB_OUTPUT
       - id: skip_check