ci: switch to jdk 21 only for the folio step

.github/workflows/ci.yml

@@ -27,16 +27,11 @@ jobs:
           go-version-file: go.mod
           cache: true
 
-      - name: Set up the JDKs
+      - name: Set up JDK 17
         uses: actions/setup-java@v4
         with:
           distribution: temurin
-          # The sidecar toolchain pins 17; the metro gradle plugin folio
-          # compiles with needs a 21 runtime. Both are installed so each
-          # gradle build can pick its own.
-          java-version: |
-            17
-            21
+          java-version: "17"
 
       - name: Set up Android SDK
         uses: android-actions/setup-android@v3
@@ -100,9 +95,16 @@ jobs:
       - name: Run tests
         run: make test
 
-      # folio's own gradle build, which `make test` does not reach. Its own
-      # step rather than part of `test` so the target developers run
-      # constantly keeps its 17 floor.
+      # folio is its own gradle build, and the metro plugin it compiles with
+      # needs a 21 runtime where the sidecar toolchain pins 17. Switching
+      # JAVA_HOME after `make test` rather than installing both up front
+      # leaves every step above this one on exactly the JDK it ran on before.
+      - name: Set up JDK 21 for folio
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: "21"
+
       - name: Run folio's unit tests
         run: make test-folio