CI: fix Rust toolchain on macOS (#7674)

Co-authored-by: Matt Mastracci <[email protected]>

.github/workflows.src/build.inc.yml

@@ -129,26 +129,33 @@
 <% endif %>
 
     steps:
-    - uses: actions/checkout@v4
-      with:
-        repository: edgedb/edgedb-pkg
-        ref: master
-        path: edgedb-pkg
-
     - name: Update Homebrew before installing Rust toolchain
       run: |
         # Homebrew renamed `rustup-init` to `rustup`:
         #   https://github.com/Homebrew/homebrew-core/pull/177840
         # But the GitHub Action runner is not updated with this change yet.
         # This caused the later `brew update` in step `Build` to relink Rust
         # toolchain executables, overwriting the custom toolchain installed by
-        # `dtolnay/rust-toolchain`. So let's just run `brew update` early.
+        # `dsherret/rust-toolchain-file`. So let's just run `brew update` early.
         brew update
 
+    - uses: actions/checkout@v4
+      if: << 'false' if tgt.runs_on and 'self-hosted' in tgt.runs_on else 'true' >>
+      with:
+        sparse-checkout: |
+          rust-toolchain.toml
+        sparse-checkout-cone-mode: false
+
     - name: Install Rust toolchain
       uses: dsherret/rust-toolchain-file@v1
       if: << 'false' if tgt.runs_on and 'self-hosted' in tgt.runs_on else 'true' >>
 
+    - uses: actions/checkout@v4
+      with:
+        repository: edgedb/edgedb-pkg
+        ref: master
+        path: edgedb-pkg
+
     - name: Set up Python
       uses: actions/setup-python@v5
       if: << 'false' if tgt.runs_on and 'self-hosted' in tgt.runs_on else 'true' >>