GitHub Actions: install clippy after toolchain

Doing it the other way around means that when we switch to Beta, Clippy
won't have been installed on Beta (even though it's already installed on
Stable)

Author: petertseng

.github/workflows/tests.yml

@@ -126,18 +126,17 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
 
-      # We actually think explicitly installing Clippy isn't necessary,
-      # as it is currently installed by default.
-      # But we'll include this step anyway just in case that changes.
-      - name: Install Clippy
-        run: rustup component add clippy
-
       - name: Setup toolchain
         uses: actions-rs/toolchain@v1
         with:
           toolchain: ${{ matrix.rust }}
           default: true
 
+      # Clippy already installed on Stable, but not Beta.
+      # So, we must install here.
+      - name: Install Clippy
+        run: rustup component add clippy
+
       - name: Clippy
         env:
           CLIPPY: true