app: [Android] set XDG_CACHE_HOME to make os.UserCacheDir work

os.UserCacheDir can't work on Android because it doesn't have
access to the Java app context. Gio programs do have access, so set
up UserCacheDir's fallback, the XDG_CACHE_HOME environment variable.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/datadir_android.go

@@ -7,6 +7,8 @@ package app
 import "C"
 
 import (
+	"os"
+	"path/filepath"
 	"sync"
 
 	"gioui.org/app/internal/window"
@@ -20,6 +22,9 @@ var (
 func dataDir() (string, error) {
 	dataDirOnce.Do(func() {
 		dataPath = window.GetDataDir()
+		// Set XDG_CACHE_HOME to make os.UserCacheDir work.
+		cachePath := filepath.Join(dataPath, "cache")
+		os.Setenv("XDG_CACHE_HOME", cachePath)
 	})
 	return dataPath, nil
 }