Update bazel-toolchains to 3.1.0
Update bazel-toolchains to 3.1.0

The version of bazel-toolchain in the current WORKSPACE file has some invalid escape sequences (https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/1756#951353b2-9c81-4819-b89e-e448b043f284). Bazel itself uses bazel-toolchains 3.1.0 right now, and those invalid escape sequences have been fixed some time before 3.1.0.

Change-Id: I013dfb1202bb2cbecd0d479e0fcd9e59a80ce929

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