Add cache step to CI jobs (swiftlang#157)

Author: mattt

.github/workflows/ci.yml

@@ -1,6 +1,6 @@
 name: CI
 
-on: 
+on:
   push:
     branches: [master]
   pull_request:
@@ -13,6 +13,12 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v1
+      - uses: actions/cache@v2
+        with:
+          path: .build
+          key: ${{ runner.os }}-spm-${{ hashFiles('**/Package.resolved') }}
+          restore-keys: |
+            ${{ runner.os }}-spm-
       - name: Build and Test
         run: swift test -c release
         env:
@@ -27,6 +33,12 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v1
+      - uses: actions/cache@v2
+        with:
+          path: .build
+          key: ${{ runner.os }}-spm-${{ hashFiles('**/Package.resolved') }}
+          restore-keys: |
+            ${{ runner.os }}-spm-
       - name: Install System Dependencies
         run: |
           apt-get update