Revert "Roll isolate go to binaries as generated by infra @ 6f3db5a82956."

This reverts commit a7ce77dd. Monitoring tells
us severely regressed isolate performance.

TBR=jam@chromium.org
BUG=644410

Review-Url: https://codereview.chromium.org/2344563003
Cr-Commit-Position: refs/heads/master@{#418639}

tools/luci-go/linux64/isolate.sha1

-2a20c5133a61d6637430b016192ebc4c0e2eae43
+cf7c1fac12790056ac393774827a5720c7590bac

tools/luci-go/mac64/isolate.sha1

-6e65d4b05d9ab86b39c7df55318ac8df36a0dcf0
+4678a9332ef5a7b90b184763afee1c100981f710

tools/luci-go/win64/isolate.exe.sha1

-1754d73e77fdc08f39dcf2b3afa31566ffec1622
+98457ff4fc79d05661fea53d2b3aff70fac90022