rustbuild: Fix source tarballs and the vendor dir

The source tarball creation step would attempt to skip a number of files that we
want to ignore ourselves, but once we've hit the vendor directory we don't want
to skip anything so be sure to vendor everything inside that directory.

Closes rust-lang#38690

src/bootstrap/dist.rs

@@ -391,6 +391,13 @@ pub fn rust_src(build: &Build) {
             }
         }
 
+        // If we're inside the vendor directory then we need to preserve
+        // everything as Cargo's vendoring support tracks all checksums and we
+        // want to be sure we don't accidentally leave out a file.
+        if spath.contains("vendor") {
+            return true
+        }
+
         let excludes = [
             "CVS", "RCS", "SCCS", ".git", ".gitignore", ".gitmodules",
             ".gitattributes", ".cvsignore", ".svn", ".arch-ids", "{arch}",