build: update to bazel v3.0.0 (#19033)

Updates to Bazel v3.0.0. Primarily the motiviation for this is
that it contains a fix for Windows `bazel run <..>`. This previously
broke since the workspace directory is no longer created in the
runfiles directory. This causes targets like `unit_tests_local` to fail.

Author: devversion

.bazelversion

@@ -1 +1 @@
-2.1.0
+3.0.0

WORKSPACE

@@ -105,9 +105,9 @@ sass_repositories()
 # Bring in bazel_toolchains for RBE setup configuration.
 http_archive(
     name = "bazel_toolchains",
-    sha256 = "4d348abfaddbcee0c077fc51bb1177065c3663191588ab3d958f027cbfe1818b",
-    strip_prefix = "bazel-toolchains-2.1.0",
-    url = "https://github.com/bazelbuild/bazel-toolchains/archive/2.1.0.tar.gz",
+    sha256 = "239a1a673861eabf988e9804f45da3b94da28d1aff05c373b013193c315d9d9e",
+    strip_prefix = "bazel-toolchains-3.0.1",
+    url = "https://github.com/bazelbuild/bazel-toolchains/archive/3.0.1.tar.gz",
 )
 
 load("@bazel_toolchains//repositories:repositories.bzl", bazel_toolchains_repositories = "repositories")
@@ -122,7 +122,7 @@ rbe_autoconfig(
     # platform configurations for the "ubuntu16_04" image. Otherwise the autoconfig rule would
     # need to pull the image and run it in order determine the toolchain configuration.
     # See: https://github.com/bazelbuild/bazel-toolchains/blob/master/configs/ubuntu16_04_clang/versions.bzl#L9
-    base_container_digest = "sha256:fd5690d000da5759121f28ccbc19ebb4545841d816bbf6a72de482cf3e7ce491",
+    base_container_digest = "sha256:5464e3e83dc656fc6e4eae6a01f5c2645f1f7e95854b3802b85e86484132d90e",
     digest = "sha256:0b8fa87db4b8e5366717a7164342a029d1348d2feea7ecc4b18c780bc2507059",
     registry = "marketplace.gcr.io",
     # We can't use the default "ubuntu16_04" RBE image provided by the autoconfig because we need