Use caching in GHA (#4)

This revealed an issue with the `venv` target due to a missing `generates` argument.

.github/workflows/go.yml

@@ -1,4 +1,8 @@
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
 name: Test
 jobs:
   test:
@@ -22,8 +26,18 @@ jobs:
       uses: actions/checkout@v4
     - name: Bootstrap
       run: go install ./cmd/dawn
+    - name: Restore cache
+      uses: actions/cache/restore@v3
+      with:
+        path: .dawn/build
+        key: dawn-build-${{ github.ref }}
     - name: Build
       run: dawn
+    - name: Save cache
+      uses: actions/cache/save@v3
+      with:
+        path: .dawn/build
+        key: dawn-build-${{ github.ref }}
     - name: Test
       run: go test ./... -coverprofile=coverage.out
     - name: Upload coverage data

cache.go

@@ -34,6 +34,8 @@ func (c *cache) get(key string) (starlark.Value, bool) {
 //     once calls the given callable if and only if key is not present in the cache.
 //
 //     The result is stored in the cache under the given key.
+//
+//     Returns the result of the call or the cached value.
 //     """
 //
 //starlark:builtin

docs/BUILD.dawn

@@ -6,7 +6,7 @@ def run_in_windows_venv(cmd):
 
 run_in_venv = run_in_windows_venv if host.os == "windows" else run_in_posix_venv
 
-@target(sources=["requirements.txt"])
+@target(sources=["requirements.txt"], generates=["venv"])
 def venv():
     """
     Creates a venv for the docs build.

init.go

@@ -3,6 +3,7 @@ package dawn
 import "go.starlark.net/resolve"
 
 func init() {
+	// Relax some Starlark restrictions.
 	resolve.AllowRecursion = true
 	resolve.AllowGlobalReassign = true
 	resolve.AllowSet = true