build: Add GitHub actions build for Windows (#8627)

This is sort of a proof of concept, but since our current Windows
builder is down this might solve the problem. It includes a change for
easier code signing (taking the certificate in a secret/env var rather
than existing already on disk), but otherwise mirrors precisely what we
already do in the build server.

build.go

@@ -15,6 +15,7 @@ import (
 	"bytes"
 	"compress/flate"
 	"compress/gzip"
+	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"flag"
@@ -1383,6 +1384,33 @@ func windowsCodesign(file string) {
 		args := []string{"sign", "/fd", algo}
 		if f := os.Getenv("CODESIGN_CERTIFICATE_FILE"); f != "" {
 			args = append(args, "/f", f)
+		} else if b := os.Getenv("CODESIGN_CERTIFICATE_BASE64"); b != "" {
+			// Decode the PFX certificate from base64.
+			bs, err := base64.RawStdEncoding.DecodeString(b)
+			if err != nil {
+				log.Println("Codesign: signing failed: decoding base64:", err)
+				return
+			}
+
+			// Write it to a temporary file
+			f, err := os.CreateTemp("", "codesign-*.pfx")
+			if err != nil {
+				log.Println("Codesign: signing failed: creating temp file:", err)
+				return
+			}
+			_ = f.Chmod(0600) // best effort remove other users' access
+			defer os.Remove(f.Name())
+			if _, err := f.Write(bs); err != nil {
+				log.Println("Codesign: signing failed: writing temp file:", err)
+				return
+			}
+			if err := f.Close(); err != nil {
+				log.Println("Codesign: signing failed: closing temp file:", err)
+				return
+			}
+
+			// Use that when signing
+			args = append(args, "/f", f.Name())
 		}
 		if p := os.Getenv("CODESIGN_CERTIFICATE_PASSWORD"); p != "" {
 			args = append(args, "/p", p)
@@ -1402,7 +1430,7 @@ func windowsCodesign(file string) {
 
 		bs, err := runError(st, args...)
 		if err != nil {
-			log.Println("Codesign: signing failed:", string(bs))
+			log.Printf("Codesign: signing failed: %v: %s", err, string(bs))
 			return
 		}
 		log.Println("Codesign: successfully signed", file, "using", algo)