Honor RESTIC_CACHE_DIR environment variable

Fix #3382: restic check doesn't obey the RESTIC_CACHE_DIR environment variable

cmd/restic/cmd_check.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/spf13/cobra"
 
+	"github.com/restic/restic/internal/cache"
 	"github.com/restic/restic/internal/checker"
 	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/fs"
@@ -146,6 +147,9 @@ func prepareCheckCache(opts CheckOptions, gopts *GlobalOptions) (cleanup func())
 	}
 
 	cachedir := gopts.CacheDir
+	if cachedir == "" {
+		cachedir = cache.EnvDir()
+	}
 
 	// use a cache in a temporary directory
 	tempdir, err := ioutil.TempDir(cachedir, "restic-check-cache-")