cmd/dist: don't copy files ending in ~ to bootstrap directory

They are editor backup files. They are ignored by .gitignore,
so they can never be real files in the Go repo.

Change-Id: I58800e6e9f939e0bd21b086243b9260bcc8cd770
Reviewed-on: https://go-review.googlesource.com/c/go/+/584675


Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/buildtool.go

@@ -113,6 +113,8 @@ var ignoreSuffixes = []string{
 	// with PGO. And as it is not a text file the import path
 	// rewrite will break it.
 	".pgo",
+	// Skip editor backup files.
+	"~",
 }
 
 var tryDirs = []string{