Add .reproxy_cache/ for remoteexec builds

remoteexec stores temporary files in a .reproxy_cache/

Change-Id: I74af0119175b11c7527613fd85f8623bacabe389
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/161440
Reviewed-by: James Price <jrprice@google.com>
Kokoro: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

.gitignore

diff --git a/.gitignore b/.gitignore
index b268054..0154d06 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,6 +6,7 @@
 /.gclient_entries
 /.gclient_previous_custom_vars
 /.gclient_previous_sync_commits
+/.reproxy_cache
 /bazel-*
 /build
 /buildtools