fix: CI caching wasn't working (#913)

This splits cache saving and restoration into two steps. Restoring
occurs in every step, while saving only occurs in a single action on
main.

.github/actions/build-cache/action.yml

@@ -2,20 +2,18 @@ name: Build Cache
 description: Cache builds
 runs:
   using: "composite"
+  # Make sure to keep these cache entries in sync with those in writecache.yml
   steps:
-    - name: Cache Go Modules
+    - name: Restore Go Modules Cache
       id: cache-go-modules
-      uses: actions/cache@v4
+      uses: actions/cache/restore@v4
       with:
         path: ~/go/pkg/mod
         key: ${{ runner.os }}-{{ hashFiles('**/go.sum') }}-go
-    - name: Cache Maven Modules
+    - name: Restore Maven Modules Cache
       id: cache-maven
-      uses: actions/cache@v4
+      uses: actions/cache/restore@v4
       with:
         path: ~/.m2/repository
         key: ${{ runner.os }}-{{ hashFiles('**/pom.xml') }}-maven
-    - name: Populate Cache
-      id: populate-cache
-      uses: ./.github/actions/populate-cache
 

.github/workflows/ci.yml

@@ -19,9 +19,7 @@ jobs:
       - name: Build Cache
         uses: ./.github/actions/build-cache
       - name: Test
-        run: |
-          find ~/.m2 -ls
-          mvn test --batch-mode
+        run: mvn test --batch-mode
   test:
     name: Test Go
     runs-on: ubuntu-latest

.github/workflows/writecache.yml

@@ -0,0 +1,32 @@
+on:
+  push:
+    branches:
+      - main
+name: Write Cache
+jobs:
+  write-cache:
+    name: Write Cache
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Init Hermit
+        uses: cashapp/activate-hermit@v1
+      - name: Build Cache
+        uses: ./.github/actions/build-cache
+      - name: Download Maven Dependencies
+        run: mvn dependency:resolve --batch-mode
+      - name: Download Go Dependencies
+        run: go mod download -x
+      - name: Save Go Modules Cache
+        id: cache-go-modules
+        uses: actions/cache/save@v4
+        with:
+          path: ~/go/pkg/mod
+          key: ${{ runner.os }}-{{ hashFiles('**/go.sum') }}-go
+      - name: Save Maven Modules Cache
+        id: cache-maven
+        uses: actions/cache/save@v4
+        with:
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-{{ hashFiles('**/pom.xml') }}-maven