Cache .cosmocc and o for github workflows

Uses GitHub’s actions/cache@v4 to store the cosmocc distribution and the
output directory between runs of the build workflow, with the version of
cosmocc as the cache key.

Upgrades to actions/checkout@v4.

.github/workflows/build.yml

@@ -1,5 +1,8 @@
 name: build
 
+env:
+  COSMOCC_VERSION: 3.9.2
+
 on:
   push:
     branches:
@@ -19,7 +22,20 @@ jobs:
       matrix:
         mode: ["", tiny, rel, tinylinux, optlinux]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
+
+      - uses: actions/cache@v4
+        with:
+          path: .cosmocc/${{ env.COSMOCC_VERSION }}
+          key: cosmocc-${{ env.COSMOCC_VERSION }}
+
+      - uses: actions/cache@v4
+        with:
+          path: o
+          key: o-${{ matrix.mode }}-${{ env.COSMOCC_VERSION }}
+          restore-keys: |
+            o-${{ matrix.mode }}-
+            o-
 
       - name: support ape bins 1
         run: sudo cp build/bootstrap/ape.elf /usr/bin/ape