Update update-dev.yml to use git instead of third-party action (#1242)

The `update-dev` action keeps failing due to unrelated histories between
the `dev` and `master` branches, and the third party action used for the
nightly merge does not include any options to allow merging with
unrelated histories. So, this PR updates the action to use `git`
instead.

This will also give us more flexibility in the future.

Couple of updates:
* I have confirmed that configuring `git` using `secrets.NIGHTLY_TOKEN`
does not expose the token in the workflow logs. See
[here](https://github.com/PennyLaneAI/qml/actions/runs/11408914907/job/31748090296#step:3:4)
for an example.

[sc-76254]

---------

Co-authored-by: Rashid N H M <95639609+rashidnhm@users.noreply.github.com>

Author: mudit2812

.github/workflows/update-dev.yml

@@ -27,12 +27,28 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          token: ${{ secrets.NIGHTLY_TOKEN }}
 
       - name: Nightly Merge
-        uses: robotology/gh-action-nightly-merge@v1.3.3
-        with:
-          stable_branch: 'master'
-          development_branch: 'dev'
-          allow_ff: false
         env:
-          GITHUB_TOKEN: ${{ secrets.NIGHTLY_TOKEN }}
+          CONFIG_USERNAME: GitHub Nightly Merge Action
+          CONFIG_EMAIL: actions@github.com
+          MERGE_HEAD: master
+          MERGE_BASE: dev
+          MERGE_ARGS: --no-ff --allow-unrelated-histories --no-edit
+        run: |
+          # This script is adapted from the robotology/gh-action-nightly-merge@v1.5.2 GitHub action:
+          # https://github.com/robotology/gh-action-nightly-merge/blob/master/entrypoint.sh
+
+          git config --global user.name "$CONFIG_USERNAME"
+          git config --global user.email "$CONFIG_EMAIL"
+
+          git fetch origin $MERGE_HEAD
+          (git checkout $MERGE_HEAD && git pull)
+
+          git fetch origin $MERGE_BASE
+          (git checkout $MERGE_BASE && git pull)
+
+          git merge $MERGE_ARGS $MERGE_HEAD
+          git push origin $MERGE_BASE