Just use git for checkout

actions/checkout is repeatedly flaky

See also actions/checkout#1951

On the other hand, plain git clone seems to work just fine and never has
connection issues checking out the default branch

Funnily enough, the YAML is shorter this way too

.github/workflows/third_party.yml

@@ -49,9 +49,7 @@ jobs:
     timeout-minutes: 60
     steps:
       - name: Checkout pydantic
-        uses: actions/checkout@v4
-        with:
-          repository: pydantic/pydantic
+        run: git clone https://github.com/pydantic/pydantic.git
       - name: Checkout typing_extensions
         uses: actions/checkout@v4
         with:
@@ -89,10 +87,7 @@ jobs:
     timeout-minutes: 60
     steps:
       - name: Checkout typing_inspect
-        uses: actions/checkout@v4
-        with:
-          repository: ilevkivskyi/typing_inspect
-          path: typing_inspect
+        run: git clone https://github.com/ilevkivskyi/typing_inspect.git
       - name: Checkout typing_extensions
         uses: actions/checkout@v4
         with:
@@ -137,10 +132,7 @@ jobs:
     timeout-minutes: 60
     steps:
       - name: Check out pyanalyze
-        uses: actions/checkout@v4
-        with:
-          repository: quora/pyanalyze
-          path: pyanalyze
+        run: git clone https://github.com/quora/pyanalyze.git
       - name: Checkout typing_extensions
         uses: actions/checkout@v4
         with:
@@ -186,10 +178,7 @@ jobs:
     timeout-minutes: 60
     steps:
       - name: Check out typeguard
-        uses: actions/checkout@v4
-        with:
-          repository: agronholm/typeguard
-          path: typeguard
+        run: git clone https://github.com/agronholm/typeguard.git
       - name: Checkout typing_extensions
         uses: actions/checkout@v4
         with:
@@ -236,10 +225,7 @@ jobs:
     timeout-minutes: 60
     steps:
       - name: Check out typed-argument-parser
-        uses: actions/checkout@v4
-        with:
-          repository: swansonk14/typed-argument-parser
-          path: typed-argument-parser
+        run: git clone https://github.com/swansonk14/typed-argument-parser.git
       - name: Checkout typing_extensions
         uses: actions/checkout@v4
         with:
@@ -291,10 +277,7 @@ jobs:
     timeout-minutes: 60
     steps:
       - name: Checkout mypy for stubtest and mypyc tests
-        uses: actions/checkout@v4
-        with:
-          repository: python/mypy
-          path: mypy
+        run: git clone https://github.com/python/mypy.git
       - name: Checkout typing_extensions
         uses: actions/checkout@v4
         with:
@@ -342,9 +325,7 @@ jobs:
     timeout-minutes: 60
     steps:
       - name: Checkout cattrs
-        uses: actions/checkout@v4
-        with:
-          repository: python-attrs/cattrs
+        run: git clone https://github.com/python-attrs/cattrs.git
       - name: Checkout typing_extensions
         uses: actions/checkout@v4
         with: