cmd/doc: properly set GOPROXY to avoid deprecation checks

This change fixes a bug that was introduced in CL 675155. Instead of
doing the two step download and run with GOPROXY=off, do the run with
GOPROXY=<download cache>:$GOPROXY, so that we use the previously
downloaded version of pkgsite as the latest.

Fixes golang#73833

Change-Id: I8803426498ab026602805d6448a130eb11458c99
Reviewed-on: https://go-review.googlesource.com/c/go/+/675576
Reviewed-by: Jonathan Amsterdam <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: matloob

src/cmd/doc/main.go

@@ -258,11 +258,22 @@ func doPkgsite(urlPath string) error {
 	// exit before exiting ourselves.
 	signal.Ignore(signalsToIgnore...)
 
+	// Prepend the local download cache to GOPROXY to get around deprecation checks.
+	env := os.Environ()
+	vars, err := runCmd(nil, "go", "env", "GOPROXY", "GOMODCACHE")
+	fields := strings.Fields(vars)
+	if err == nil && len(fields) == 2 {
+		goproxy, gomodcache := fields[0], fields[1]
+		goproxy = "file://" + filepath.Join(gomodcache, "cache", "download") + "," + goproxy
+		env = append(env, "GOPROXY="+goproxy)
+	}
+
 	const version = "v0.0.0-20250520201116-40659211760d"
 	cmd := exec.Command("go", "run", "golang.org/x/pkgsite/cmd/internal/doc@"+version,
 		"-gorepo", buildCtx.GOROOT,
 		"-http", addr,
 		"-open", path)
+	cmd.Env = env
 	cmd.Stdout = os.Stderr
 	cmd.Stderr = os.Stderr