Switch to shared fast-submodule action

Author: dtolnay

.github/actions/checkout/action.yml

@@ -3,16 +3,16 @@ name: git submodule init
 runs:
   using: composite
   steps:
-    - uses: ./.github/actions/fast-submodule
+    - uses: dtolnay/.github/.github/actions/fast-submodule@master
       with:
         submodule: rust
     - run: git -C rust log --all
       shell: bash
-    - uses: ./.github/actions/fast-submodule
+    - uses: dtolnay/.github/.github/actions/fast-submodule@master
       with:
         working-directory: rust
         submodule: library/backtrace
-    - uses: ./.github/actions/fast-submodule
+    - uses: dtolnay/.github/.github/actions/fast-submodule@master
       with:
         working-directory: rust
         submodule: library/stdarch