Revert pull_request_target (#225)

`pull_request_target` has not been the drop-in replacement it promises
to be. Revert its addition and disable build jobs for external PRs.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 2 people

.github/workflows/development.yml

@@ -12,8 +12,6 @@ jobs:
         python: ["3.9", "3.13"]
     steps:
       - uses: actions/checkout@v4
-        with:
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
@@ -49,8 +47,6 @@ jobs:
         python: ["3.9", "3.13"]
     steps:
       - uses: actions/checkout@v4
-        with:
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
@@ -86,8 +82,6 @@ jobs:
         python: ["3.9"]
     steps:
       - uses: actions/checkout@v4
-        with:
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
@@ -123,8 +117,6 @@ jobs:
         python: ["3.9", "3.13"]
     steps:
       - uses: actions/checkout@v4
-        with:
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
@@ -160,8 +152,6 @@ jobs:
         python: ["3.9", "3.13"]
     steps:
       - uses: actions/checkout@v4
-        with:
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
@@ -202,7 +192,6 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
       - name: Set up Python
         uses: actions/setup-python@v5
         with: