switch back to old goma backend in linux-rel

We have occasional compile failure due to backend error.
https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.try/linux-rel?limit=200
Let me switch back to use old goma backend until we fixed the error.

Bug: b/135145905, 976362
Change-Id: I6da5b5a94fdd9dcb2b52cf1a21aa7da9a3f82ca2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1665774
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670293}

infra/config/cr-buildbucket.cfg

@@ -4182,7 +4182,7 @@ buckets {
     builders {
       mixins: "linux-try"
       mixins: "linux-xenial"
-      mixins: "goma-rbe-prod-j150"
+      mixins: "goma-j150"
       mixins: "builderless"
       name: "linux-rel"
     }